According to the Mauna Kea Visitor information center:
2-wheel drive vehicles are not permitted above the Visitor Information Station. A 4-wheel drive vehicle with Low Range is STRONGLY RECOMMENDED. About 300 yards beyond the station, the pavement ends and the next 4 and a half miles are a steep graded-gravel road. You should check with your rental car company to see if you are allowed to travel on Maunakea.
Is it actually difficult to drive on this road with an AWD car in practice?
I've driven on this road in January 2021 and I can confidently say its not particularly difficult with an AWD SUV. You'd probably be fine even with a 2WD that has a good enough clearance, although I'm not sure if the security guard at the visitor center gate will let you through without a car that looks like an SUV.
But be aware that rental company insurance will not cover you on the stretch past the visitor center, so you will be on the hook if something happens to the car while you're driving.
I successfully drove a compact rental car to the top of Mauna Kea about 15 years ago when I visited the Big Island. As far as I can recall, the car did not have any particular AWD or 4WD capabilities; it was a Nissan Sentra or Toyota Corolla or something like that.
I also don't recall there being any official rules about two-wheel drive cars being barred from the road at the time, like there are now. That said, I did violate my agreement with the rental agency by taking the car past the visitor center.
