I've listened to a few interviews with Jesse Ventura and he's obviously not that bright, and pretty crazy. I'd be inclined to believe the other guys version of things in pretty much any dispute, never mind one where the other guy is a war hero. Also, I've heard Ventura say a lot of stupid, inflammatory things, and if he was a little drunk it wouldn't surprise me if he started saying things that he wouldn't say if he was sober and in front of a microphone.
That said, the one compelling piece of evidence on Ventura's side is that apparently there's a picture of him from the next day, and there's no sign that he had been punched in the face by a huge warrior like Chris Kyle. Ventura is an old man, and was apparently on blood thinners at the time, and Kyle was in his prime. You'd think if he got knocked on his ass, he'd show some sign of it.
Ventura is almost always full of shit as far as I can tell, but in this case it sounds like he's telling the truth, and maybe Kyle drunkenly mistook someone else for Ventura (or maybe deliberately conflated an actual guy with Ventura to increase book sales.)