Do you mean XHHW instead of XHHN?
The THWN has a thinner jacket is for wet locations which the 2008 clarifies inside pipe outside as a wet location.
Dont see a XHHN listing in the NEC
There is a listing for XHHW which is also listed for wet locations however the outer jacket is thicker than THWN.
THWN is most likely easier to pull and handle both good conductors for your purpose.