I was able to passthrough the integrated GPU to a Windows 8 Preview domU while simultaneously have another Windows 8 domU with a passthrough AMD HD 5450 PCIe. Pressing my monitor input button allowed my to switch between both domU.
Of course this involved no graphic output from the dom0, 2 keyboards, 2 mice, SSH access from another computer and passthrough of USB controllers to domUs.
This on Debian Wheezy, Xen 4.1.3 and Linux kernel 3.2.0 and 3.5-trunk.
thanks a lot for this info. So from your experience the integrated GPU will apparently not automatically be disabled, when a discrete GPU is plugged in the PCI Express x16 slot, as stated by Kevin_Intel (thanks for your answer as well, Kevin). Are there any special BIOS settings you had to take care of, so the integrated GPU will stay enabled? What BIOS version are you using?
I'm actually planning to not passthrough the integrated GPU so it can be used by dom0. Do you see any reason, why this shouldn't work?
Wouldn't it be possible to use a hardware KVM-switch to use only one mouse, keyboard and monitor for your setup?
Thanks again for your answer!
Using both 0049 and 0050 BIOS, under Configuration > Video you can "configure how the IGD is enabled or disabled based on the priority of other video devices". I have on "Always Enable" and use a PCIe AMD HD 5450 as I mentioned before.
As we speak my workstation is running Debian on dom0 using the i7-3770's IGD and Windows 8 Preview on the discrete GPU. I've also tested the scenario in previous post: dom0 without display + IGD displaying Windows 8 Preview domU + discrete HD 5450 powering the other Win8 domU.
Haven't succeeded in passing the HD5450 to a Linux HVM though I managed to so with the IGD.
There are report on the web about people using KVM switchs but personally I don't have experience with them.
Setting up PCI/VGA passthrough requires researching for VT-d hardware and reading lots on Xen. Regarding VT-d hardware, DQ77MK and i7-3770 (non-K) are working for me but have a look at Xen's wiki for more info.