Xenon is the base version of the bot and is free for everyone. More information can be found here and here.
Xenon and Xenon pro are sharing the same database. That means backups from Xenon can be used with Xenon Pro and vice versa.
Xenon Pro extends the features of Xenon and adds some new cool additions. There is no need to keep Xenon on the server when using Xenon Pro. More information can be found here.
It costs $2.99 per month and can be bought on Patreon.
Xenon Turbo is connected to a separate database. That means backups created with Xenon and Xenon Pro can not be used with Turbo.
Xenon Turbo does not extend the features of Xenon or Xenon Pro. It concentrates on the backup feature and is able to save and load a lot more than the other two tiers. It doesn't have any of the Pro features, beside the copy command. You need to keep Xenon Pro on your server to use them. More information can be found here.
It costs $14.99 per month and can be bought on Patreon.