Mobi and AZW3 (aka KF8) are both at their core Palm DB files. AZW3 are basically Mobi files for backwards compatibility purposes, with an added compiled ePub f

Mobi and AZW3 (aka KF8) are both at their core Palm DB files. AZW3 are basically Mobi files for backwards compatibility purposes, with an added compiled ePub file for improved layout options. Kindles can read both formats, with newer Kindles using the HTML/CSS from AZW3 files. As far as I know, the only options to get an ePub onto a Kindle convert to an AZW3 before putting sending it to the Kindle (or manually convert to a Mobi file before sideloading it). And those conversions aren't perfect, I've absolutely had technically correct ePubs end up as a confused mess when sent to a Kindle. Though, these days most publishers seem to be targeting whatever subset of ePubs Amazon supports in their conversion pipeline.