update, for those with the same problem: I figured out how to get xv output on xawtv. I had to load a "v4l" in the "Modules" section of my XF86Config-4. Evidently, this enables xvideo extensions for the v4l driver.
Oddly enough, when I run with xv enabled, and then I quit xawtv (or mplayer, or any other program viewing through my tv card), the audio from the tv card just keeps on going... very strange.
When i run xawtv -noxv, this doesn't happen (but of course I'm back to where I started, without fullscreen).