You’re running a Dom 0 Xen kernel with a nvidia graphic card?
And you’re frustrated by the low-res X (vesa) when running on X? Want a higher res X cuz you have a kick-ass Nvidia video card?
sax2 -m 0=nv
should resolve all your troubles.
note: Don’t expect 3D acceleration yet. Nvidia is currently working on a xen video driver.