I have read all his novels and know his cast of characters well but I really know very little about the historicity of Sven Hassel.
The stories are written as first person stories and even his Wikipedia page claims he participated on all fronts, something which sounds very unlikely to me.
The Danish writer Erik Haaest has been disputing Hassel's claims for many years.
From Sven Hassel, 3 - Controversy:
Haaest writes that Sven Hassel is actually Børge Villy Redsted Pedersen, a Danish Nazi who never served on the Russian front. According to Haaest, the author spent the majority of World War II in occupied Denmark and his knowledge of warfare comes second-hand from Danish Waffen-SS veterans whom he met after the end of the war. Haaest also alleges that Hassel's first novel was ghostwritten and when it became a success, he employed his wife to write the rest of his books. Haaest claims that during the war period, Pedersen/Hassel, was in fact a member of the HIPO Corps or Hilfspolizei, an auxiliary Danish police force created by the Gestapo, consisting of collaborators.
Can any historian shed some light on this?
What's known about him is what was written in his books and on his book jackets, which hasn't quite been the same story in all versions. This study of his books (Swedish), also takes a very skeptical view and shows (p 81) how many of his books occur at the same time but in different places. It references Hassel's explanation for this in Morgenposten, claiming his books took place over 'hours or days' and that he was being moved around; the study on the other hand, claims all his books (by that point) took place over periods of at least a month and a half. It also mentions quite a lot of other stuff on his biography, and not only using Haaest as a source.
The name "Hassel" is certainly a pseudonym since he apparently got sued in Denmark by the Hassel family who couldn't find any relation to him, so his books there were published under the name "Hazel" instead.
The reference above as well as the obituary of him in Jyllands-Posten, state that the journalist Georg Kringelbach investigated and questioned his past in a radio show in 1963, and also found that he was Børge Willy Pedersen Arbing, and had worked for HIPO. Haaest's book on Hassel came out in 1976.