Do a long listing (ls -l) of /boot and post the output. When the xen package is installed the full name of the hypervisor is soft linked to xen.gz. For example, ls -l /boot/xen.gz lrwxrwxrwx 1 root root 17 Jan 18 19:21 /boot/xen.gz -> xen-4.6.0_06-1.gz >From the error, the above link cannot be made. This would be a problem with your filesystem.