As it says, been offered a killer deal on a pair of wheels that have an 11speeed XD driver. Can I change this and fit a shimano hub or will I need to change hub completely ?

That depends on the hub

Giant branded P-TRX’s, the composite ones with I believe DT swiss hub ?

No idea in that case and just because it has DT internals doesn’t mean much. I’d try asking a Giant dealer if a part is available.

If you’re running 11 speed, a SRAM GX cassette is about the same price as an XT. but is a good chunk lighter.