On XenServer, a user can run both Linux and Windows virtual machines, and like physical machines, it can be set to boot from various sources. However, when a Linux virtual machine is set to boot from DVD, it can fail with the following error message:

To allow the Linux virtual machine to boot from DVD, set the virtual machine to start in Recovery Mode, as shown in the following screen shot:

Problem Cause

Unlike Microsoft Windows, which uses device drivers for paravirtualization, Linux virtual machines have a paravirtualized kernel. During the installation of Linux, it is actually running as a Hardware-Assisted Virtual Machine (HVM) and has access to DVD just like Windows.
When the Linux installation is complete, a Xen kernel is swapped in. However, as the paravirtualization is kernel based, this causes issues with accessing the DVD on boot, as the kernel is not loaded.