When you talk 'guidelines' you're really getting into publishers' territory. For specific requirements, check publisher pages and see if something is specified for a certain line or call.
If you're just wanting a general definition of what is a paranormal romance, that's a bit easier.
A paranormal romance will first of all be a romance. There will be a growing relationship that is the main thrust of the story and it will end in a Happily Ever After (HEA), just as it would in any other romance.
The second element of a paranormal romance is the paranormal aspect. There is a broad spectrum of creatures or abilities you can use for this, from vampires and werewolves to magic and ESP. Whatever your paranormal aspect is, it should also be intergal to the storyline. While the paranormal element is usually attached to a character, it can be a force outside the romantic leads, but some publishers prefer that one of the leads be a paranormal creature or have a 'gift.'
Specifics like book length and heat level are publisher preferences.
Does that help?