If I understand your question correctly, what you are after is not possible. The reason for this is that encryption can protect data while it is transit or at rest, but not while it is being processed. As the comments stated the host OS must be running in order to run the guest, so you cannot encrypt the Ubuntu partition that is also running your VMs.
A solution is to either make a dedicated machine as a hypervisor (which if you run without a GUI will consume less resources and need less patching) or to encrypt any sensitive files on your host using file encryption. An example of a program that can encrypt individual folders or the entire drive (which will not work for this use case) is Veracrypt. After creating the secured folder/container with Veracrypt, you could decrypt it only when you need access to the contents and running your VM, leaving it encrypted whenever you use the VM so that if a VM escape exploit does occur the attacker can still not read your data.
EDIT: Just making it clear that this answer is based on the below quote from the question;
say for example: UBUNTU(host machine):->run VM(Arch) and have Ubuntu be completely Encrypted whilst in the Arch VM?
You can encrypt your host/hypervisor with disk encryption to protect data during transport or rest, but you cannot have the host/hypervisor encrypted while it is actually running VMs.