Webkit is KHTML.
Webkit briefly was KHTML, but the past 7-8 years of development clearly distinguished one from the other. Apple sends the changes they've made back to the KHTML folks, but as I understand it, KHTML hasn't simply adopted all of Apple's changes lock-stock-and-barrel.
Not so many people would agree that Webkit is the best HTML rendering engine (Gecko, KHTML or Opera, anyone?).