Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person (Omniscient) / First Person (Central Narrator) / First Person (Peripheral Narrator)
Whoa there, Jazz—way to hog all of the narrative techniques.
Jazz, like the music it's a shout-out to, doesn't play by the rules. There's the presence of a shadowy narrator in Jazz who pops up occasionally to speak in the manner of a first person peripheral narrator, telling us what s/he sees the other characters do and what, exactly, s/he thinks about them. We don't know much about this shadowy mystery narrator, other than that s/he is looking for love in all the wrong places. And we don't find that out until the very end of the novel.
In other places, we get first person narration from a central narrator. Violet, Joe, Felice, and Dorcas all get a turn to speak into the "I" microphone, and they tell us everything about what they're thinking and feeling. The reader gets the impression that we have VIP access into their heads—and is that VIP room ever messy: Things get free-association-y and stream-of-consciousness-y fast.
Most of the book, however, is told by a disarmingly traditional third person narrator, who has an omniscient vantage point. Pages go by without a crack in the solid façade of third person omniscience, making the reader feel at ease and in control… and then boom, the narrative spirals into a bizarre first person party.
It's jarring, sure, but it's also interesting. You know, kind of like jazz music is can be jarring and interesting. Jazz music has no problem stringing the listener along with a basic melody for a while and then putting a spin on that melody that makes the listener smile with bewildered awe and pleasure.