If you’re using Paravirtualised (PV) guest Virtual Machines on Citrix XenServer, then it’s not immediately obvious how to boot the guest from a CD/DVD should you need to. In the “Boot Options” tab of the Properties for the Virtual Machine, the “Boot Device” drop down menu will be greyed out, with no obvious way of enabling it.
The actual method of booting a Virtual Machine from a CD/DVD is a little less obvious – go to the Virtual Machine in question, switch to the console tab and select the CD/DVD from the drop down list at the top. Make sure that the Virtual Machine is shutdown, then click on the “VM” menu, then “Start/Shut down” and select “Start in Recovery Mode”. The Virtual Machine will now boot from whichever CD/DVD you’ve selected.