If the configuration file of a PVHVM virtual machine or template lists a virtual CDROM drive as an IDE device, that is as /dev/hda, dev/hdb, and so on, the virtual CDROM is not available inside the guest. To enable the CDROM drive inside the guest, it must be defined in the vm.cfg file as a paravirtual device, that is /dev/xvda, /dev/xvdb, and so on.

This is described in How are Virtual Disks Managed? in the Oracle VM Concepts Guide.

Bug 14000249 and 18012889