You answered your question. One is a passion another just a hobby. I have lots of hobbies but few of them do I do with great passion.
I will use the example of photography. It started out as just taking pictures then I thought it would be good to take a class, which moved me to get a better camera, which pushed me toward taking a more serious class, which led to taking a full college course that lasted 4 years. That is a passion.
I've fished all my life and that is also a passion, my ex said it was more like insanity, just because I own 20 poles and reels and a boat.
I like to build things but that is just a hobby, I love to do special effects makeup and I love to build haunted houses I have studied both but they are still hobbies.
If it consumes you and gets your blood pumping, if it fills your thoughts to the point that you can ignore minor things like eating or sleeping it is a passion. It doesn't have to be that way all the time but when you get into it a passion blinds you to distractions