Those murdered in the Holocaust are known as “kedoshim”. What is the source for this title?
The term kedoshim does not refer only to those killed specifically in the holocaust, but all those that were killed because they are Jews. Since they were killed and they gave their lives up Al Kiddush H-shem, they are called kedoshim. There are a number of reasons for this, wether because thier level in shomayim is very high, therefore they are kodesh kadashim, or because their death was in the form of a korban, which is also called kodesh, so they are called kodoshim. See Pirush Ballei Hatosefos Shemos 28-37. Also see Aderes Eliyahu Devorim 1-15, Maharil Teshuva 98.