Allegory has to rely on a precise one-to-one relationship between story-element and meaning. A novel is more democratic: the meaning is not dictated or determined by the author, but is something that emerges in the space between the story and the reader's mind. Myth is a different matter. C.S. Lewis would say that a myth is a story that has the same force, the same effect, the same meaning, in whichever form we encounter it: it's independent of its telling. Again, a novel is something slightly different. A novel might tell a mythical story, but it would not need to be mythical in order to be a novel.