I have made and uploaded a two-part video series on VGA pass through to Windows XP HVM domU to Youtube.
Here are the Youtube links:
Basically Windows XP guest operating system has direct access to PCI Express x16 graphics card but in a reduced functionality state. The videos show that the passed through display card cannot found enough resources it can use. The device status for nVidia Geforce 8400 GS on my Intel Desktop Board DQ45CB is:
This device cannot find enough free resources that it can use. (Code 12)
I could not run any 3-D video benchmark tests at the moment. I have tried Sisoftware Sandra Lite 2009.SP4 and Passmark Performance Test benchmark suites. 3-D video benchmark tests in both suites cannot be started at all. Work on direct graphics card access by Windows virtual machines is still in progress but this paves the way for heavy 3-D gaming in Windows virtual machines in the future. Besides processors, graphics card virtualization will become mainstream in the not-too-distant future. The technology requires Intel Virtualization Technology for Directed Input/Output (VT-d).
In addition to graphics card, you also need to pass through the USB keyboard and USB optical mouse to the Windows virtual machine in order to use it. However, USB pass through is currently not possible on Xen 3.5-unstable with Intel applied VGA passthrough patches. The code is still experimental and under constant development.
Special thanks to Xen virtualization developers and engineers at Intel Corporation for making VGA passthrough possible.
Thank you very much.