http://www.xenlens.com/boot-a-guest-vm-from-cd-or-dvd-in-xenserver/ copied for posterity
In order to boot from cd or dvd you need to change the guest virtualization type from HVM (fully virtualized) to PV (paravirtualized).
xe vm-param-set HVM-boot-policy="BIOS order" uuid=[uuid of your vm]
After you have booted from dvd, change back to fully virtualized mode:
xe vm-param-set HVM-boot-policy="" uuid=[uuid of your vm]