First of all i want to say that performance with xen was never an issue, it works great for me, it feels almost native, but so does kvm.
I moved away from xen initially because, like i mentioned in previous posts, it doesnt support ( as of today ) s3 resume ( on my machine at least ), and using
cool'n quiet crash my host, i cant even get to X before it crashes, i never had any issue using kvm (KVM IS LINUX!)
Second, kvm/qemu is moving way faster than xen/qemu, xen is still using qemu 0.12 i believe ( xen 4.3 supports upstream qemu but its buggy as hell), which is like 4 years old, so its missing a lot of features from upstream (GL USING AUDIO PASSTHROUGH ON WIN7 X64), and xen supposed "vga-passthrough support" involves tons of hacks that have never been merged in like 3-4 years.
Intitially what i wanted to achieve was to "partition" my machine ( each one independent as possible from the other ) using 2 cards, so for example
if wanted to say play on my windows vm and build a kernel on my dom0 using make -j128 (lol) the host wouldnt slow down the vm, or the other way arround, so i though hell why dont i passthrough my sata controller to the vm, dedicate 6 of my 8 cores to it ( and leave 2 for the host + 4GB of ram ). Well unfortunately i could never get rombios from xen to boot from it, i also tried to force it to load the optiomrom for the controller ( from a file ), and it did load but the boot table got corrupted and it wouldnt boot anything, then i found out i could use a edk2 duet image to recognize my controller on ahci mode, and boot windows 7 x64 uefi, and yeah that worked, but power management and audio passthrough still didnt work.
So then i found kvm, and it could do eveything everything xen did + more ( seabios does boot my sata controller, audio passthrough works w00t ) but PRIMARY vga passthrough didnt work, until a month ago, when i found a message on qemu-devel implementing this upstream ( without 3'rd party unmantained hacks ) so im happy with it and its working great.
EDIT: i'd like to add that gplpv drivers are horrible! HORRRIBLEEEEEEEEEE OMG SRSLY THEY MAKE ME CRY, never had a problem with virtio drivers though.
Edited by nbhusain - 5/6/13 at 1:02pm