So far if the node is deployed with 18.04, then only that particular OS version can be deployed to KVM pod.
Unfortunately, it is not under our plans to supporting a KVM pod with non-Ubuntu OS.
But what about different Ubuntu release?
It does make sense to have the ability to install a 19.04 pod on 18.04 node or 16.04 pod on 19.04 node.
Currently we have only tested KVM host (pod) deployment on Ubuntu 18.04. It’s possible to use a more recent version of Ubuntu as a KVM host, but most people running Ubuntu in production do so using the LTS release (18.04 being the latest), so that is where we have focused our development and testing efforts.
Alternatively, you can use MAAS to deploy the machine, set up libvirt yourself, and manually add the pod. If you do it this way, you should configure bridges in MAAS for any network you want to attach to, and be careful not to change the bridge names.
Once you have deployed a KVM host, you can run virtual machines using any OS you choose (assuming you have downloaded the image in MAAS).