Starting with version 2.80.0, calibre has the ability to sideload books to Kindle devices in KFX format and to view the metadata of sideloaded KFX books present on a Kindle device.
When your Kindle is connected additional icons will appear on the calibre toolbar.
"Send to device" sends books from your calibre library to your device and
"Device" shows the books currently present on the device.
"Remove books" will delete books from a device if activated while in device view.
Use the "Configure this device" choice in the "Device" dropdown menu to adjust the priority of different formats. If a book has multiple formats that are checked on the list then the one highest on the list is delivered. Conversion to the highest priority format will be done automatically if none of the checked formats is available for a book. Move "kfx" to the top if you want it to have precedence.
See the
calibre User Manual for more information.