The Predator has superior strength to an Alien and is larger. The Alien films rarely show the alien fighting in close quarters: usually it grabs people and they're too scared to move. No one ever even tries to escape when that happens. The only instance we see of an Alien actually fighting a human in CQC is in Alien 3 vs. Dillon (spelling?). And Dillon survives for a long time, getting his face scratched slowly away. That doesn't seem like a very good CQC fighter. The original predator fought Dutch, a man several time Dillons' physical superior, in CQC, and absolutely destroyed him. He didn't even look as if he was trying! The hunter was nonchalantly throwing blows at Dutch so as to inflict maximum pain and enjoy the victory. If he had been putting 100% brutal force and effort in, with unrelenting blows, Dutch would have been dead VERY quickly.
What can be concluded from this? The predator is 1) larger (i.e taller and heavier), and 2) stronger (the alien tail is pretty strong, though...but arms and legs on a predator are definitely stronger). And if the xenomorph tries to headbite him, it would have to get in close, which the predators' superior strength could easily remedy.