In order to boot from cd or dvd you need to change the guest virtualization type from HVM (fully virtualized) to PV (paravirtualized).
$ sudo xenstore-read vm | xargs -n1 basename
et HVM-boot-p olicy="BIOS order" uuid=[uuid of your vm]
After you have booted from dvd, change back to fully virtualized mode:
et HVM-boot-p olicy="" uuid=[uuid of your vm]
Read more about difference between HVM and PV: