I'm currently using Xen PV on CentOS 5 with my domU's as flat files running on a hardware RAID controlled (write cache enabled) formatted with XFS. On the dom0 I can get about 500MB/s in a 2GB dd write from /dev/zero however on the domU's I'm lucky if I get 10MB/s (it is usually around half that).
I've tried changing the disk scheduling to NOOP on the domU's, changed some mount parameters and tweaked the performance allocations of both the dom0 (prioritize CPU) and domU's (increase RAM and VCPU allocations). None of these steps have produced any noticeable change in performance.
My instinct here is that it is not a hardware problem, due to the solid performance of the dom0. Any ideas on what might be causing this problem? I'm considering moving to LVM based domU's.