I've been running various operating systems in KVM virtual machines for some years. It's fun, but the problems of viewing the desktop in a window have always been a little annoying. Even in full-screen mode (on its own desktop in the host's window manager), the linux host is sometimes evident.

But I figured out how to run virt-viewer in its own virtual console such that there are no telltails from the host except the CTRL-ALT-Fn key sequences. For example, to access my Windows 10 virtual machine, I start it with virsh as before and start the viewer like this.

Notice that I'm not even running a window manager. The -k switch on virt-viewer puts it in full screen with a minimal interface. It really feels like the other OS owns the machine. And I can log in and out of the host and the virtual machines independently.

Gosh, this is fun! The only downside is that I haven't figured out how to exit virt-viewer without literally running "kill" from a terminal in a different virtual console.

enable the ctrl-alt-backspace X exit.

I thought about that, but I didn't know how to do it. And I only want it to work on X servers running virt-viewer, not window manages on the host. (At some point, I'm certain to get confused about which desktop I'm looking at.) It required a deep dive into the unseemly world of X. Also, this this answer helped.

I created file /etc/X11/CtrlAltBksp.conf:
And my script that starts X specifies that configuration file:
Now, with this setup, CTRL-ALT-Backspace tears down an X server serving virt-viewer, but my host desktop(s) still ignore it.

This keeps getting better!