Build xen-hypervisor 4.16 and xen-tools on Ubuntu 18.04 or 20.04 for PVH build from source
Ubuntu 16.04 comes with xen-hypervisor 4.6, the next LTS-version Ubuntu 18.04 ships xen 4.9. Updates for meltdown will only published for xen 4.8 and 4.10.
To protect your system with PVH against meltdown you have to build your xen-hypervisor from source.