AZW3 — Kindle KF8 format. Use for all
modern Kindles:
Paperwhite (all generations), Kindle Fire / Fire HD, Kindle Oasis, Kindle Scribe, Kindle Colorsoft, Kindle app (iOS/Android/PC).
MOBI — Older format. Use for
vintage Kindles (released before 2012) that don't open AZW3:
Kindle Keyboard (K3), Kindle 1, Kindle 2, Kindle DX, Kindle 3G.
Not sure? Try AZW3 first.
See Wikipedia: Amazon Kindle.