What type of freehub body is required for Rival 12-speed road cassettes?

You will need a rear hub with an XDR driver body. Replacement XDR driver bodies are available from most wheel manufacturers to retrofit 11-speed road rear hubs. If you currently have an 8, 9, or 10-speed drivetrain, upgrading your wheelset or rear hub to one that incorporates compatibility to an XDR driver body will allow you to install any eTap AXS groupset to your bike.