While attempting to install the Xen Hypervisor on a machine of mine using a perfectly fresh Ubuntu Server 16.04 Xenial installation the machine would freeze early in the booting process. Specifically I would see just a few lines of output from Xen which ended in the following line:
Loading Initial Ramdisk...
After a bit of hunting I discovered the issues is that Xen doesn't work properly with EFI installations. To remedy this you'll need to reconfigure the machine to boot in Legacy BIOS mode (the procedure to do this changes drastically with each computer).
What I did to fix it was reboot and when selecting the boot media I selected my thumb drive from a list titled Legacy instead of a list titled EFI (e.g. the drive appears in two locations).
Once that was done I reinstalled the OS, upgraded the installation, reinstalled Xen, and rebooted just fine.
$ sudo apt update $ sudo apt upgrade # apply new kernel $ sudo reboot $ sudo apt install xen-hypervisor-amd64 $ sudo reboot
If you want to keep your existing installation and get rid of Xen then you'll want to reboot, select Ubuntu from the GRUB menu (Xen has been made default), and uninstall Xen.
$ sudo apt remove xen-hypervisor-amd64 $ sudo reboot