Boot a bad kernel, reboot the system, boot a good kernel.
From the good kernel: journalctl -k -b0 > good_kernel.txt journalctl -k -b-1 > bad_kernel.txt
Boot a bad kernel, reboot the system, boot a good kernel.
From the good kernel:
journalctl -k -b0 > good_kernel.txt
journalctl -k -b-1 > bad_kernel.txt