I have seen it. The pretext of the show is Carr starts off by saying that no subject is out off bounds and the jokes will get more distasteful as the show goes on, until either the crowd stops laughing or the material ends. The holocaust joke is the last and therefore the worst taste joke about the most taboo subject to make jokes about. Context as always is everything.
From the Guardian-
The show opened with Carr telling viewers: “Before we start, a quick trigger warning. Tonight’s show contains jokes about terrible things. Things that may have affected you and the people you love. But these are just jokes. They’re not the terrible things.”
In the special, Carr sought to explain what he said was the context of the joke, saying it was “edgy as hell” and had an educational value.
“It’s a joke about the worst thing that’s ever happened in human history, and people say ‘never forget’, well this is how I remember,” he said.
“There is an educational quality. Like everyone in the room knows, 6 million Jewish people lost their lives to the Nazis during the second world war. But a lot of people don’t know, because it’s not really taught in our schools, that the Nazis also killed, in their thousands, Gypsies, homosexuals, disabled people and Jehovah’s Witnesses.”
It is interesting to note that lot's of his fellow comics are saying that, although they find some of his stuff distasteful, they defend his right to say it.
It is also interesting to note that the people who want him censored, are also the ones who were advocating leaving people in small boats to drown in the english channel.