Haig Tchamitch , Mar 06, 2009; 10:46 p.m.
The TAv mode essentially allows you to set the shutter speed, and the aperture while the camera adjusts for the exposure using the ISO. Let's say that you are shooting some kids playing.. so you need the shutter speed to be at 1/250, and you also would like a shallow depth of field so as not to get background clutter.. so you set your aperture at f4. At this point these are constants with the Tav mode.. and as the light changes the camera compensates with the ISO to get the proper exposure.
The Sv mode just sets your shutter speed to be a constant, and your ISO is set by you, the camera adjusts the aperture to get the proper exposure.