SVF is a file format which can be used for programming FPGAs and other JTAG operations.
XSVF is a binary version, which seems to now be deprecated by Xilinx.  Here are some useful resources:

* http://www.clifford.at/libxsvf/
* https://www.xilinx.com/support/documentation/application_notes/xapp058.pdf
* https://www.xilinx.com/support/documentation/application_notes/xapp503.pdf