mhmm... I was wrong when I said there was no problem anymore. It seems like GLX fails.
glxinfo gives the following : "Error: couldn't find RGB GLX visual" and then some values.
glxgears gives : ""Error: couldn't get an RGB, Double-buffered visual"
any gl app fails. also, nvidia-settings segfaults when you try to see GLX information.
But, twinview is working and that was what I needed
Still, I need to reboot into a non-xen kernel If I want to play a GL game.