Darwin/XNU is not supported at the moment.
panicall has
ported
(video)
syzkaller to Darwin/XNU and that has found more than
50 bugs including
CVE-2018-4447 and CVE-2018-4435 mentioned in
Apple security updates.
Darwin/XNU is open-source and has
KASAN,
but no KCOV at the moment (though not required for intial support).
PureDarwin may be used to create VM images suitable for fuzzing.