NetBSD (and OpenBSD)

Build the the kernel per upstream documentation and then copy it to either of:

/pool/$(KVM_PREFIX)netbsd-kernel.$(uname -r)
/pool/netbsd-kernel.$(uname -r)

During upgrade the stock kernel will be replaced with the above.

Linux, Fedora, Debian

These systemd assimilated distros all boot the test kernel directly from:

$(KVM_POOLDIR)/$(KVM_PREFIX)linux-upgrade.vmlinuz
$(KVM_POOLDIR)/$(KVM_PREFIX)linux-upgrade.initramfs

These files are re-created whenever ./kvm transmogrify is run. To boot a different kernel, replace the above (but remember, rerunning ./kvm transmogrify (also ./kvm clean and ./kvm uninstall) will revert things to the old kernel.

Would a better approach be to install the custom kernel into the linux-upgrade domain?