Can THHN Wire be used in PVC under ground.

No, THHN is listed (Article 300, Table 310.13(A)) as dry or damp areas.

Earth is considered a wet area. Conduit, below ground level is also considered to be in a wet area.

THHW, THW, THW-2,THWN, THWN-2 and TW are acceptable for wet areas.

Check the wire you may have, most THHN is dual rated as THHN/THWN