As I understand it Kellerbier is not a necessarily a wheat beer, it is an unfiltered lager. Similar to a marzen/octoberfest/vienna lager but without filtering/finings. Sierra Nevada produces a kellerweiss which is in fact a wheat beer. There was a thread posted not too long ago that discussed kellerbier and the creation of recipies for homebrewing. The search function should point you to it.
Hefe and Wit are both wheat beers. Hefe is composed primarily of malted wheat and pilsner/2-row. Wit uses raw or torrified wheat. Each has their own specialized yeast strain(s) as well. If you were to attempt to create a classic hefe with anything other than a weizen yeast, you would not have a true hefeweizen. Also, wit generally involves spices such as coriander and bitter orange peel that you would not find in a traditional hefeweizen.
I'm sure I've left something out and someone will come along shortly and correct us both