I'm pretty sure you'll be disappointed by the results anyway. I spent some time making a few tests lately.
The iRedOS CD won't include a so called "paravirtualized" CentOS kernel. That's not a fault of iRedOS. The "paravirtualized" CentOS kernel is only shipped within its DVD version. Many distributions behave like that.
Basically, don't try to run a fully virtualized CentOS on the Xen Server. It won't make sense in any case. "Paravirtualized" is the way to go whenever possible.
Here's what I did. Grab the "Xen Center", the M$FT Windows client for Xen Server. Use the supplied templates within the program. There are CentOS V.5.3, V5.4 templates ready to go. The template process includes the possibility of remotely accessing file repositories. It's pretty straight forward for any "power user". Install only a bare, very minimal selection of CentOS initial packages; on V5.3 that means 172 packages I think. Complete installation.
Then access your new CentOS install via SSH. Follow all steps included in iRedMail script install guide.
That's pretty much it! Enjoy iRedMail!