It's not explicitly said whether Viktor Krum kissed Hermione Granger, however, Harry Potter remarks internally that he knew for a fact that she had snogged Viktor Krum while he was at Hogwarts, presumably at the Yule Ball or sometime thereafter. This information is revealed when Ron Weasley gets in a fight with his younger sister after finding his younger sister making out with her boyfriend in the portrait hole. She yelled at him that he's only behaving that way because he hasn't snogged anyone whereas all of his friends have, including Hermione.