Xen wiki page, I was under the impression that GPU passthrough only works with HVM guests. But maybe my impression is wrong. When I get a chance, I'll try again.
I was only able to get GPU passthrough working with the Ubuntu 12.04 HVM guest using whatever default version of Catalyst (not the update) that is found in the Ubuntu 12.04 repository. Considering that the open source drivers work fine with Ubuntu as dom0, I think it's odd that GPU passthrough does not seem to work with Ubuntu 12.04 HVM guest when using the open source Radeon graphics drivers.
Since I hope to get GPU passthrough working with a greater variety of Linux guests (whether with XCP or vanilla Xen), I submitted a "suggestion" to Xen's new uservoice page requesting that the Xen Project work more closely with the developers of open source video drivers to enable GPU passthrough to Linux guests using open source drivers. See http://xenorg.uservoice.com/forums/1...g-open-source-
If you're also interested in getting GPU passthrough working with a greater variety of Linux guests, I hope that you'll consider adding your vote and comments to my suggestion.
Oh, I found this thread while searching for examples of GPU passthrough using KVM. If I'm not mistaken, GPU passthrough using KVM will be much easier using qemu 1.5 combined with kernel 3.9. Daily builds of Ubuntu 13.10 (featuring kernel 3.9) are now available. At present, only qemu 1.4 is available from the Ubuntu 13.10 repository, but hopefully version 1.5 will be available soon. Once it is, Ubuntu 13.10 would seem to be a good prospect for testing GPU passthrough using KVM. I like Xen. But if KVM provides better support for GPU passthough to Linux guests, I'll consider switching to KVM.