I use Xen for Host and most of my Guests are Linux

Although I do have a few Windows Guests - performance is not an issues

One of my Clients is using SLES 11 SP4 KVM and with Windows Guest and looking at performance

He is currently using Fedora / RedHat virtio drivers is this does improve Windows performance for both HDD & Network

Novell no longer provide Windows Virtio drivers with SLES 11 (not even with OpenSuse)

He has asked me if Novell's drivers are better than Redhat/Fedora's
Novell's drivers will cost $700 so... (multiple sites so cost is important factor!)

client is asking are there PERFORMANCE benefit - not just Supported !

I'm not in a position to comment as I have not tried Novell's Virtio drivers

My own tests with KVM & Windows - adding the Redhat /Fedora drivers does make a lot of improvement to performance
and seem to work Ok without issues