Hi,
For xVA I would suggest the "Open Source Risk Engine" by Quaternion. It is based on Quantlib.
If ISDA Simm is the thing you need then you might be interested in the free implementation of ISDA Simm by Acadia Soft
https://github.com/AcadiaSoft/simm-lib This library is written in Java. In consequence I allow myself to promote the use of the finmath Java library
https://github.com/finmath that provides also Adjoint Algorithmic Differentiation capabilities. I think this is a good starting pont.