In this context, "passionate" represents a stronger degree of involvement than "interest."
Examples:
"I am interested in the Romantic composers, but I am only passionate about Beethoven."
"I find Henning Mankell's detective novels interesting, but I'm not really passionate about them."
"I told Weight Watchers, 'You can take away my potato chips, you can take away my bacon, but don't mess with my doughnuts--I'm passionate about them!'"