I'm starting to wrap my head around Feynman diagrams, and the idea of "real" vs. "virtual" particles, but one area where this distinction seems to break down is in describing Hawking radiation, where virtual particles become real particles through interaction with gravity.
How would one explain the difference between real and virtual particles in an interaction which produces one from the other? In the example of Feynman diagrams, are these on-shell or off-shell? When do they become real in the interaction? How is the fundamental "what-goes-in-must-come-out" rule allowed to be broken?