[HarryT]

If the ePub book contains embedded fonts, the fonts will be present in the converted AZW3 book and it will look exactly the same. If the book doesn't contain embedded fonts, it will request a font from the device which will be mapped as closely as possible to one of the fonts present on the device, exactly as happens with an ePub book. There's no difference in behaviour between the two formats.

If a book contains embedded fonts, you can switch between displaying the book using its embedded fonts (which the Kindle calls "Publisher Fonts") or using your own font selection.

I really don't care about fonts in the slightest, so I'm probably not the best person to ask about how good the Kindle's fonts are. They look absolutely fine to me, but it's not something I'm fussy about.