Here is how I understand these various sub-genres:
Low Fantasy, while I believe technically can be modern day, typically is not. How I think of low fantasy is basically a fantasy story dealing with ground level characters, and little to no traditional (ie dragons, wizards, etc) fantasy elements. Low fantasy typically won't deal in good vs. evil, world-in-the-balance events, at least not as its main focus. Sword-and-Sorcery, rogueish characters are the norm, and typically magic may exist but isn't commonly seen, and its existence is often doubted by the characters. Joe Abercrombie and Scott Lynch are the authors I think of when I think of this sub-genre.
Urban and Contemporary Fantasy are pretty similar, if not interchangeable. They feature magic in a contemporary setting. Urban fantasy is usually exclusively set in a city, though the term has become so common it pretty much just means fantasy tropes within our modern world. American Gods, like you said, would definitely fall under the definitions of both urban and contemporary fantasy, as well as the Dresden Files by Jim Butcher or the Anita Blake novels by Laurell K. Hamilton.
Magical Realism, as I think of it, usually denotes a literary novel that has elements of fantasy. The line between 'literary' and 'genre' fiction is arbitrary and dumb, and is slowly dissolving, but as it is now, it's mainly a pretentious way of describing any kind of fantasy book. Salman Rushdie and Gabriel Garcia Marquez are more on the literary side, as is Murakami, though he is closer to the dividing line between 'literary' and 'sci-fi/fantasy', and someone like David Mitchell straddles the two genres pretty equally.
Paranormal seems to typically be used as an adjective adding a descriptor to another genre. Like your example, Twilight's main element, or at least its main draw, is the romance, and paranormal colors the characters and setting. Twilight could easily be described as a contemporary fantasy, or a paranormal fantasy, as well. It's all in what aspect of the work you want to emphasize.
Knowing only what is in this topic regarding your book, I would say urban fantasy is probably the closest definition. You could likely even go with 'Paranormal Fantasy' as (at least to me) paranormal sort of implies paranormal elements in an otherwise realistic setting. (I don't think anyone would consider the ghost army in Lord of the Rings as a paranormal element because basically everything in the world is fantastical.
Anyway, that is my understanding of the various subgenres, as somebody who reads quite a lot from each of them.
Hopes this helps at least a little bit!