There's been more than one discussion thread on this subject, and the summary answer is:
- It's possible to do with a para-virtualized driver in the guest - that is, a driver that takes the calls for graphics commands and passes them to Dom0, where a "real" graphics driver executes the actual intention of the call.
It is not trivial to write such a driver, but it's nowhere near as hard as it would be to try to get a very advanced native driver to work in a guest environment.
Of course, if you have two different graphics cards in the machine, Dom0 can be given one card, and one of the guest domains can be given the other card, thus you can use the native driver straight off (at least in fully virtualized mode).
