In my opinion, such people or groups are not motivated by any desire for the truth. They claim that they have real doubts about the evidence for the Holocaust, but I believe that they only really exist because they have some reason to want to defend the Nazis or to dislike the Jews.
People who are Holocaust deniers are generally (in my opinion) A) very conservative Christians (Mel Gibson) who have religious reasons for hating Jews B) people who sympathize with fascism or C) Muslim extremists (Ahmadinejad) who also have religious or geopolitical reasons to hate Jews and want Israel weakened.
Without accusing any specific groups of people, anyone who deliberately refuses to accept factual historical evidence is either (a) extremely hard-headed; (b) intellectually challenged; or (c) defending an agenda that conflicts with the topic at hand. Germany's attempts to extinguish the Jewish populations of the European nations it conquered has unlimited factual documentation and thousands of survivors who can still attest to the attrocities perpetrated by the Nazis. To deny that the Holocaust happened shows ignorance or gullibility tantamount to idiocy. One would have to totally disregard the literally millions of examples of documentation--both written and human--that the Holocaust has sadly provided.
It is a very unbelievable trend to deny facts that are so undeniable and proven. Certainly they must have their reasons, but I believe that the deeper reason is perhaps to detour the attention from the things that really need to change.
Since to some people change is a difficult concept, it is better to deny that something has or had to be done about people's attitudes towards those they were brainwashed to detest just because of someone's weird ideas.
Conservative Christians do not deny the Holocaust and they certainly do not hate Jews. Christians, conservative as well as others, believe the Jews are God's chosen people and they are as horrified by the Holocaust as others, probably more so because they believe that all humans are created in God's image and to take a life is the worst of sins. Also, the actual facts regarding Mel Gibson is NOT that he denies the Holocaust. He questioned the extent of the Holocaust because, in his view, he didn't see how Hitler could have exterminated six million Jews. His view is NOT accepted by Christians, conservative or not. Anyone that visits the Holocaust museum in Washington D.C. (or anywhere else, for that matter - there are many such museums worldwide) could have no doubt that the Holocaust happened. Also, some of the defunct concentration camps are also visited by thousands of people worldwide. Many people who hid Jews during the Holocaust were conservative Christians. Please check your facts.
It is my belief that many people allow themselves to be blinded based on what they chose to be the truth. I had the bad experience while living in Germany to meet several German mothers at a park one day. My children were playing and I was studying a book on the Holocaust for a class that I was taking at a German University. One of the women stated to me,” You don't believe that rubbish, do you?" I then explained to her that it had happened. I told her I had even visited the camps. I did not want her to know my parents were Holocaust survivors at the time. The other woman proceeded to tell me how wonderful Hitler had been to the Germans and that he only had taken from the Jews what had belonged to the Germans but the Jews had been sent to other countries to live. I was amazed that they truly believed this.
I have read about the leader of Iran and his statements, but I know that there is no changing the mind of an individual who refuses to see.
The truth is validated by public records, artifacts, and the many survivors who shared their stories about their family members and their own lives during the Holocaust. People like my parents (Holocaust survivors) spent years telling their stories in schools and teaching about the love of humanity in an effort to prevent further atrocities. We can not change the way other people chose to think, we can only share what we know and the rest is up to them.
I am personally not aware of any group of people who claim that occurrence of holocaust is not a historical fact, or that the there is not enough evidence to prove its occurrence beyond doubt. If any such group exists, I doubt if their belief is based on systematic historical studies. I once read in a newspaper, a few decades back, about a group of people who maintained that the earth is not round but flat. These people had their own reasoning to support their assertion. But no serious scientist bothered to take them seriously, or counter their arguments. I believe, if a group exists that claims that holocaust is not a historical fact then, they are living in a world created by their imagination, and the best way to deal with them is to ignore them.