Very few people are Holocaust deniers in the sense of denying that the systematic murder of Jews and others took place; there's too much evidence to demonstrate that it did, so to deny the Holocaust outright is to put you in the same grouping as evolution-deniers - people living outside of the borders of factual reality.
What people such as David Irving (said to be, I've heard, all else aside, a superb historian) do is quibble over the exact numbers and exact points of detail, saying for example that it wasn't 6,000,000 Jews, it was (for example) 5.3 million - as though we're all supposed to go, oh, well, not as bad as we thought, then ...