I found this useful post about doing some optimizations for your Xen hosts and guests:
Unfortunately, I had some trouble finding where to make the mods on the host. The location you will need is /boot/extlinux.conf. I think this may be the RedHat location...
Also, these lines are very long, and if you edit them with nano, for instance, you may screw up the file. I did this once, and the machine would no longer boot. You have been warned. So use vi. (Weren't you always looking for a good excuse to learn vi?)
I just added the kernel param to the 'xe' section.