Unikernels, LibraryOSes (MirageOS,Gramine and Occulum)

You can choose the kernel used by vms with (change VMNAME and VALUE accordingly):

qvm-prefs VMNAME kernel VALUE

You can do it with gui too, in the Qubes Manager.

If you want to install an arbitrary kernel in a vm, check out my guide: How to install older version of kernel on R4.1 - please help with a clear straightforward guide? - #2 by BEBF738VD