In a lot of cases the only difference in a comic book and a graphic novel is the length but even then, that's an important difference. Think of the difference in a short story and a novel. A graphic novel has time to more fully develop the story.
In a lot of cases though, there's a difference in the quality of the story as well. Comics tend to be geared more toward kids. A lot of graphic novels are as well but a lot are much more grownup.
I'm not a fan of either although I've always had some interest in them. I learned to read from comics. I'd figure out as much from the pictures as I could and ask for help with the words and by the time I entered the first grade I'd already read the Tarzan novels and most of the Hardy Boys novel as well as other books. When I was in college working as a taxi driver there was extra money to be made answring requests for Spanish speaking drivers so I did the same thing; I bought Spanish language comic books, which are really more like graphic novels with stories similar to the sort of thing you'd see on TV on Lifetime and Hallmark. With them and a dictionary I was able to learn enough Spanish to handle those calls.
Since then I've read a few comics every year and a couple of graphic novels. Most suffer from the same sort of imaturity but there are a few that do rise above that. Art Spiegleman's "Maus", a graphic novel, won a Pulitzer. And even some comics are good. If anyone wants a really amazing literary treat take a look at Will Eisner's "The Spirit". These are beautiful stories; silly sometimes, yes, but never without real humanity and maturity. In fact they're the reason newspapers have comic pages.
Back to the topic, though, the Kindle is too small to read comics comfortably. I'm sure it can be done by those with good vision but it's far more comfortable on a larger screen. I did try reading a grapic novel on a Kindle and about 20 pages into it I moved it to a tablet. And even then I wasn't happy till I got a full HD tablet.
After reading those Spanish comic novels I've always wished I could find something like that in English. I can manage the Spanish if I have a dictionary but it's more work than fun. And I haven't used my Spanish for probably 20 or 30 years so I'm not sure I could even do it now. ut that is a medium with real promise. I'd like more of it if it's makers would take it seriously.
Barry