Just curious, but why does Canon use Av and Tv to designate aperture and shutter priority? What do they stand for?
Understand first that Canon's designers are Japanese and both speak and think in Japanese. How a concept thought of in one language translates to another often varies with the translator.
The generally accepted terms that Canon chose to use long ago in a galaxy far away (read: before there two more than 2 or 3 cameras with such options and absolutely no common convention had been established) are:
Av = Aperture Value, where you choose a fixed aperture, more commonly termed Aperture Priority.
Tv = Time Value, where you choose the shutter open time (aka shutter speed), more commonly termed Shutter Priority.
BTW, this would be a better poll if the list of options was complete. There are two very distinct forms of "Program" out there. One is the PHD (Push Here Dummy) version usually selected by setting some green symbol ("set it on green and go) and which locks out all or most overrides. The second is a variable Program or Program Shift mode most often selected by setting a non-green P and which allows altering the f/stop and shutter speed combo with a simple turn of a control wheel.
Personally, I use the latter Program/Shift mode most of the time. The end result is, for my shooting, a much better choice than either of the more limiting A and S modes. I rarely take more than one shot (often 2-3 actual images in succession) of any one subject. As a result, a chosen f/stop or shutter speed from one shot is not likely to be appropriate for the next. The means there's no advantage to the camera retaining that selection. Every shot requires me to think of the best balance of shutter speed and aperture to fit the new situation, hence P is the most functional choice. I very often shift the chosen settings a bit. When I need to deviate from the meter's opinion of the correct exposure I usually do it by altering the framing and trapping the exposure before restoring the final framing. In other situations I use the EV compensation. I use other modes on occasion when they are better fits to the situation (Panoramas needing fixed f/stop and fixed exposure, HDR needing fixed f/stop and varying exposure, ...).