Ivanhoe is clearly an adventure. With its jousting, witchcraft trials, and daring rescues, how much more excitement can one book contain? There is also plenty of family drama: Cedric can't forgive his son for leaving England to go fight in the Crusades without his permission.
The novel is historical fiction, since it was written in 1819 but set in the 1190s. It's not an entirely realistic depiction of the Middle Ages, but that's not really a requirement of historical fiction.
Ivanhoe also has plenty of romance, of both the happy (Ivanhoe-Rowena) and unhappy (Ivanhoe Rebecca; Rebecca-Bois Guilbert) varieties. In short, Ivanhoe encompasses a lot of different genres, all of them adding up to a thrilling tale.