KFX is a new Kindle format from Amazon. Very little is known about this format at this time. X seems to be Roman Numeral 10 thereby skipping 9. There is also an extension change to .AZW8 for final products.
This new format is released with the Kindle paperwhite 3 and others. It includes
- a new enhanced typesetting engine that includes soft-hyphens
- the new Bookerly font.
- a new image format with higher compression called JXR
- Page Flip: A multi-page thumbnail view that reflects the current font and margin settings.
- Table viewer: Data tables in books can be viewed and magnified independently of the other book content.
- Kindle Interactive Edition: These are like Print Replica books, but can have additional reader-activated media.
- Comics: Comics delivered in KFX format support page and panel viewing modes and double-page spreads.
- Kindle in Motion: These books have images and auto-playing video at fixed locations on pages with the main text flowing around them.
This seems to be a binary version of JSON. In this respect it seems to be a follow-on from AZK. KFX seems to be derived from a intermediate form called KDF which can be built using the latest beta version of Kindle Previewer.
 Creating KFX
This MobileRead forum discussion describes a Calibre plugin. The following quote comes from the forum. "The purpose of the KFX Conversion Output plugin is to convert e-books into KFX files that can be sideloaded to newer Kindle apps and devices. This allows books obtained from sources other than Amazon to be read with Enhanced Typesetting." This technique makes use of the Amazon Kindle Previewer version 3.
 KFX Content
An example of the top level:
B00UZSTXL4_EBOK/amzn1.drm-voucher.v1.cd614ffd-319f-4b9e-99b7-238c6fe78f1e.voucher (DRM voucher) B00UZSTXL4_EBOK/B00UZSTXL4_EBOK.azw (encrypted main container) B00UZSTXL4_EBOK/CR!6AS14VN94H79K68RG9EKT09T41N0.azw.res (resource container) B00UZSTXL4_EBOK/CR!SNRJN8W7KX07Z8MVGN4AMWXSFR3E.azw.md (metadata container)
 Kindle for PC support
Starting with version 1.19, Kindle for PC will download books in KFX format and display them with enhanced typesetting.
The various files that make up the KFX format are all present, but named differently compared with an e-ink Kindle. Here is the same book after downloading to both platforms.
Kindle Paperwhite 2:
- Perfect State (Kindle Single)_B00UZSTXL4.kfx (encrypted main container)
- Perfect State (Kindle Single)_B00UZSTXL4.sdr/assets/voucher (DRM voucher)
- Perfect State (Kindle Single)_B00UZSTXL4.sdr/assets/metadata.kfx (metadata container)
- Perfect State (Kindle Single)_B00UZSTXL4.sdr/assets/attachables/CR!AX7PNNAM616C90FBFYKZ53GVC0QK.kfx (resource container)
Kindle for PC 1.19:
- B00UZSTXL4_EBOK/amzn1.drm-voucher.v1.cd614ffd-319f-4b9e-99b7-238c6fe78f1e.voucher (DRM voucher)
- B00UZSTXL4_EBOK/B00UZSTXL4_EBOK.azw (encrypted main container)
- B00UZSTXL4_EBOK/CR!6AS14VN94H79K68RG9EKT09T41N0.azw.res (resource container)
- B00UZSTXL4_EBOK/CR!SNRJN8W7KX07Z8MVGN4AMWXSFR3E.azw.md (metadata container)