Ubuntu 16.04. Linode Tokyo still uses XEN according to their blog ‘linode tokyo are sold out’, so my VPS is not upgraded to KVM.
There are some articles in Linode Docs, but for XEN, some are marked deprecated.
But this one is still useful: Custom Compiled Kernel with PV-GRUB on Debian & Ubuntu.
title Ubuntu 4.4.0-45
kernel /boot/vmlinuz-4.4.0-45-generic root=/dev/xvda ro quiet