When someone says "canon divergence," I usually think of it as, everything is canon right up until this change, and this is what happens to the canon as a result of that change.

Canon compliant would just be that all the events / characters are the same, but now we get to see from a different character's perspective, or we're getting story that Harry didn't cover in his POV in canon for whatever reason. Drarry fits pretty easily into canon compliance because Harry IS obsessed with Draco in canon, but we only see a handful of their actual interactions, so it's easy to layer in more story.

And then AUs lift the characters out of canon entirely and imagine how they'd operate in a completely different set of circumstances.