These definitions are a tad confusing and are even somewhat arbitrary, as they often depend on the particular test design used to measure them. Some of these "thresholds" appear to be interrelated, yet it is not known exactly how. For instance, the lactate threshold and the ventilatory threshold are concomitant during exercise, but lactic acid production does not appear to be entirely responsible for the non-linear increase in ventilation. Medical patients who lack respiratory chemoreception also fail to exhibit a ventilatory threshold in spite of the fact that they experience acidosis. Conversely, people lacking in myophosphorylate enzyme cannot produce lactic acid during exercise, yet they do exhibit a ventilatory threshold.
With all that in mind, blood lactate measurements are probably the best objective method of assessing state of effort, whether or not you buy in to the idea of a specific "lactate threshold." Subjective assessment (running by feel in a so-called "steady state of effort") is an even better means of finding this same level of exertion, but if you are not sensitive enough to regulate yourself by feel, blood lactate readings can serve you pretty well.
The "CV" referred to here likewise has several possible definitions (or even several possible values), depending upon the protocol used to determine it. But it, like the other terms, really represents a level of exertion. As with the theoretical "lactate threshold," you can find it by getting in tune with your body's sensory signals. But why run at this pace? Rather than choosing an arbitrary speed and giving it a name, what are its benefits? How much time should you spend there? How often? At what times of the year? Those are some of the more prudent questions.
My background and subsequent research is in methodology rather than exercise physiology, although I have spent some time in the lab working with the metabolic cart and I try to maintain a cursory reading knowledge of human physiology. Basically, I'm the "how-to" guy and not the "here's why it works" guy; consequently, excuse any inaccuracies which I might spew forth regarding exercise science. Perhaps some of the real physiologists who frequent the forum might weigh in to correct me if I'm mistaken. Phoenix, jtupper, others?
Having offered that disclaimer, one substantial benefit of running at just above the LT appears to be increased expression of monocarboxylate transporters. There are several isoforms of MCTs, but for our purposes as runners, MCTs transport lactate across plasma membranes, making it available for oxidation in Type I skeletal muscle fibers, for glycogen synthesis in Type II skeletal muscle fibers, and in cardiac muscle. Many skeletal muscles in humans are not homogeneous; that is, they contain fibers with both oxidative and glycolytic properties. In these, different MCT isoforms (specifically, MCT-1 and MCT-4) are present in proportion to the oxidative or glycolytic nature of the fibers. Running at certain levels of exertion (principally, slightly above the LT and just up to the point of hyperventilation) will produce enough available lactate to enhance expression of MCT-1.
What pace are we talking about here? You are trying for a pace which has you operating slightly above (faster than) your maximum steady state of effort. In numerical terms, this will normally be about 8-12 seconds per mile faster than your theoretical "LT pace" (assuming you know this value or can sense it by feel). The effect of this pace will be to slightly exceed the lactate threshold while avoiding a lowering of the blood pH. This gives you a good opportunity to process or "clear" lactate while staying out of trouble.
This is almost assuredly the type of speed used by Lydiard's runners as they performed some of their controlled time trials of 3-6 miles. It is now generally recognized that a time frame of closer to 20 minutes is best for a continuous run at this pace, although Lydiard's athletes were certainly at this effort level during some stage of a time trial. Other runners undoubtedly independently discovered such an effort level prior to (and since) Lydiard's arrival on the scene.
Intermittent running (segment running or "interval" format) at this type of speed or effort level is also desirable, as this format allows you to amass more total time at the chosen workload, thereby providing a greater stimulus for improvement. My own experience with this pace began in high school in the 1970s, when I had a regular running route which lent itself to running three bouts of about 15 minutes at a time between water breaks at gas stations along the way. Initially, this routine arose with a bathroom break at one of the gas stations, and I got used to stopping there (and at the next station) for water (and for tightening shoelaces and perhaps some stretching) on subsequent runs. I would also often run this route continuously, although not as fast. I later learned through trial and error that 3 x 15 minutes was about the maximum amount of total time that it was prudent to spend at this effort level - just on the safe side of struggling.
Whether you run continuously or in segments, this pace should be maintained until the onset of hyperventilation; that is, until you feel as though your breathing pattern is no longer steady and you feel the need to slow down or stop in order to for your breathing to "catch back up." During intermittent running, a brief recovery period between bouts allows this and permits you to resume the pace without struggling. Continuing to work in a state of hyperventilation will probably not have a significant additional bearing on up-regulation of the MCT isoforms which are present in oxidative muscle fibers, yet it will certainly reduce the amount of time you are able to spend at this pace. Moreover, respiratory distress is accompanied by metabolic acidosis, which tends to promote inefficient movements if continued very long. The effect of hammering yourself in this fashion is the same as that of overracing, as it eventually leads to deterioration of fitness. In other words, the risks of operating very long in respiratory distress outweigh the benefits.
Up-regulation of the MCT isoforms present in the glycolytic muscle fibers may occur as a result of acute flooding of the muscles with lactate (the MCT system seems to actually be pH-dependent). This training does also up-regulate MCT-1 in the oxidative fibers (and does so more dramatically than moderate-intensity running will), but with fewer additional benefits (see next paragraph) and more risks than are incurred by running just above the LT for sufficient duration. The specifics of (and the desired frequency for) this high-intensity work is a story for another day, although I have alluded to some of it in posts regarding 800m and 1,500m sessions.
Obviously, running at any given pace can confer more than one benefit. By running slightly above the LT for adequate time, you will work your respiratory muscles in addition to your prime mover muscles, motor units responsible for fine motor control will adapt for function as you stay in a steady, distress-free rhythm, you will precipitate a greater reliance on fat as a fuel (this also results in finer control of acid/base status), and so on. The benefits are numerous. The question of exactly why something "works" for you, while interesting to some people, is therefore not always 100% answerable.
As an analogy, imagine a researcher of jigsaw puzzles who spills the jumbled pieces of a 500-piece puzzle out of the box and onto a table, then announces that he's discovered an extremely important fact; to wit, one of the pieces is solid red! Rather than actually assembling the puzzle, the researcher continues to underscore the value of the color of more individual pieces, perhaps even ignoring their shapes (the shape of a piece usually being a more pertinent bit of information). Of course, we know that seeing the colors can assist in putting the puzzle together, but overanalyzing the colors and overstating their importance amounts to little more than flogging a dead horse.
With that in mind, you should focus on assembly of your running "puzzle" rather than on unwarranted categorization or excessive scrutiny of its individual "pieces."