Goal: I would like to install my main Linux distro on top of KVM, equivalent to dom0 in Xen hypervisor parlance. I know that running an OS on top of a hypervisor is not uncommon and that KVM has been baked into the Linux kernel since 2.4 or so. The question is how to configure the system so that it does so automatically on boot. Ideally, the bootloader would start KVM before mounting the disks and booting into the OS.
Question: How does one accomplish this?
As you noted, KVM is part of the Linux kernel (hence the K).
Your dom0-equivalent hence is Linux kernel that you boot. KVM depends on a Linux userland to set up VMs.
There's no KVM without a host Linux.
You could invent a Linux distro that only contains a initrd with enough user land to set up the VMs you need. The practical challenge will be making sure you can hand over the graphics card used at boot to the guest. I've yet to make that work on a PC; and I guess that's what you'd want with a "main" OS.
But honestly, I don't know exactly how that would differ positively from established solutions like OpenStack; something with high privileges needs to coordinate things. After all, running VMs also ideally requires an orchestrator caring for connecting to storage, configuring external and internal networking, initiating snapshots and doing migrations. In that spirit, OpenStack has backends for both KVM and Xen, if I'm not mistaken.
If you're really just looking for something slim that can start and stop VMs very rapidly, maybe firecracker is for you.
dom0.