Currently, Xen doesn't support any direct hardware access
from fully virtualized domains [aka HVM domain => AMD-V (SVM,
Pacificia) or VMX (Intel VT, Vanderpool, etct)].
If there was support for IOMMU in Xen (and in hardware),
and the relevant software architecture to support it is present, then in theory
you could pass the entire hardware component to the HVM domain. This means the
netire nVidia/ATI/3DLabs/S3/etc graphics card, not a window thereof. To get a
window, you need some software to manage it, and it would be easiest to
implement that as a paravirtual driver that sends drawing commands to the owner
of the actual hardware (Dom0 usually).
Migration of fully virtualized domains is also not yet
supported. Only way to do that is a full shutdown and startup in the new
location. There is work in progress for this, initially to save/restore the
domain, which is "slow migration [but faster than shutdown+start]".