Would I use PBr3 or HBr in this synthesis?
As @permeakra has said neither reagent will get you the stereochemistry you want in the product. The HBr route goes through a carbocation which scrambles the stereochemistry completely.

PBr3 will invert the stereochemistry, then SN2 -CN addition will invert it again, giving you the wrong product. 

You need something that turns the -OH group into a leaving group while preserving the stereochemistry, so that the CN can displace it by a SN2 process. This will invert the stereochemistry giving you the product, after hydrolysis, you want.

I suggest forming a sulfonate, either tosylate, triflate or (my choice) mesylate. This should be displaced smoothly by cyanide under the DMF conditions you refer to.