FMCs and XMCs are both mezzanine cards mounted on a corresponding base board, but with a different field of application.
FPGA Mezzanine Cards are intended to enhance and customize functionality of an FPGA implemented on the base board. These mezzanines offer SerDes functionality as well as flexible and variable I/O to the programmable logic. Thereby, an FMC has no standardized bus structure.
XMCs – also referred to as Switched Mezzanine Cards – are direct derivates of PMCs and support a PCI(e) bus structure. They are designed to support PCI Express or other high speed serial communication in a more robust and smaller cross-platform form factor.