I installed Kernel 5.12.10-1 from current testing. It appeared in xen.cfg, which I edited to make it ithe default.
Booting up again I got a complaint that the system could not start the virtual screen. I was not able to get into the system.
As usual, when this happens, I used another Linux to edit xen.cfg to point back to an earlier, working, version of the kernel.
How do I debug a problem kernel? Also, how do I find any other earlier kernels that might work? I used kernel-latest and kernel-latest-qubes-vm to get this one.