I've been playing with Xen for quite a while - having everything virtual is a godsend - however when you hit performance problems it can be a nightmare. After recently following some guides on Xen DomU installation I noticed that performance in the Disk IO area was just horrible - so I did what anyone in my shoes would do and wrote a quick guide that will squeeze MUCH more performance out of your Xen installations than following many of the existing (and outdated) guides.
This method works so well because it keeps all blocks aligned on your RAID5 array. If you do things differently, then you will get multiple writes as parity is updated on your array causing a huge penalty to performance.
Head over to the Xen Performance Guide