/u/mikedash was so amazing as to link my MM on Holocaust Denial but for more info, also check this answer of mine on the history of Holocaust denial and its prevalence

Always more to say, but our Monday Methods discussion of holocaust denial is a really good and thorough introduction that addresses both your questions.

Holocaust Denial and how to combat it

If after reading through that you want more information about the development of holocaust denial, you might find it helpful to search through threads about some of the leading deniers, among whom David Irving certainly takes pride of place.