Realism, Historical Fiction, Biography
Above all, The Hours is a work of realism. Michael Cunningham does his best to capture the everyday thoughts and feelings of relatively ordinary people living relatively ordinary lives.
Of course, one of the novel's three protagonists isn't quite as ordinary as the others, and that's where elements of historical fiction and biography come in. In order to depict a single, believable day in the life of Virginia Woolf, Cunningham had to do his research, and the fictionalized Woolf that he has created here has elements of both fact and fiction.