What is the XL configuration in PSLV?

The XL configuration of PSLV is the upgraded version of the standard PSLV. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Indian Space Research Organisation [ISRO Notes for UPSC]
2. Mars Orbiter Mission (MOM) – A Complete Overview of Mangalyaan Mission