Heh, I just looked at xbmc. What a horrible, horrible, pig!
Its pure bloatware if I've ever seen any.
Note that there are a couple of settings in the settings menu that might help with your tearing... sync video to refresh rate, and sync refresh rate to video (or something like that)... these should hopefully stop the tearing... or would if the program wasn't just such a bloatedly slow pig. I suspect that your tearing problem *could* be related to the slow bloatedness.
Did I mention that it is a bloated slow hog of a program?
Do yourself a favor and use mythtv instead.