What is the difference between a love story and a romance novel?
It’s equivalent to the difference between a "legal thriller" and a "techno-thriller." In that instance, both novels include many of the same elements: suspense, good and bad forces pitted against each other, scenes that build to a major plot point, etc. But aside from the obvious, those novels are in different sub-genres and the sub-genres have different requirements. For instance, legal thrillers generally have a court room scene on center stage, techno-thrillers use the world or a city as their setting. Legal thrillers explore the nuances of law, techno-thrillers explore the nuances of scientific or military conflict.
The same situation applies with romance novels and love stories. Though both have romantic elements, the sub-genres have different requirements. Love stories must use universal characters and settings. Romance novels are not bound by this requirement and characters can be rich, famous, or people who lived centuries ago, and the settings can be exotic. Love stories can differ in theme, romance novels have a general theme—"the taming of a man." And finally, romance novels usually have happy endings while love stories are not bound by this requirement. Love stories usually end tragically or, at best, on a bittersweet note.