The X-Carve is a cosmetic fork of the Shapeoko 2 which since its introduction has had Wide Makerslide added to replace the double extrusion X-axis.
The Shapeoko 3, Carbide 3D’s current product was a clean slate design by Edward R. Ford (who is now a partner at Carbide 3D having left Inventables) applying all the lessons learned from previous models to create a better, albeit larger, heavier and more expensive machine.
Both are opensource designs (the SO3 has an NC clause), with files available at: https://wiki.shapeoko.com/index.php/Parts#Source_Files — one limit is the custom extrusions, the carriage extrusion for the X-Carve would have to be replaced w/ plates and spacers as were used on the SO2, while there isn’t a compatible extrusion for the custom SO3 extrusions, folks have worked up similar extrusions using V rails (one person made an SO3 clone from scratch).