Darwin/XNU is not supported at the moment.
panicall has ported ([1] (video), [2]) 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.