How to install Xen Project on Ubuntu Server 18.04.3 LTS?

The pertinent section relating to /etc/network/interfaces looks like this:

auto lo eth0 xenbr0 
iface lo inet loopback

iface xenbr0 inet dhcp   
    bridge_ports eth0

iface eth0 inet manual

Which in Netplan's /etc/netplan/config.yaml would look something like:

network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: no
  bridges:
    xenbr0:
      dhcp4: yes
      interfaces:
        - eth0

Bear in mind that your interface names will probably have changed to match the newer, more consistent, naming scheme, so eth0 will probably more likely be enxNNNNN or enoN or some such.

Then you do the usual sudo netplan generate to check, and sudo netplan apply when you're sure it's correct.