They are somewhat close in that they are both thermoplastics. PVC is obviously polyvinyl chloride, where TPO is thermoplastic polyolefin. They are generally manufactured in the same manner, they both are heat weld, and they come in the same thicknesses. The PVC membranes are soft membranes to begin with, but become stiff as they age. TPOs start off stiff, and remain that way. The big difference is that TPOs, unlike PVC membranes, don’t become brittle due to loss of the plasticizers from prolonged exposure to ultraviolet radiation, and TPOs are cheaper to manufacture. So, consider TPOs to be just like PVCs, except they don’t become brittle with age like PVCs, and they are cheaper to make.
FYI, they are making TPOs that are peel-and-stick with welded laps, fleece-backed TPOs for mopping down with asphalt, and of course they too use heat-weld laps.
I hope that answers your question. Just think of TPOs as being similar to PVC, but not having problems like the old Trocal roofs had with regard to plasticizer loss. Of course, Trocal is the worst example of PVC, and those membranes were made before they found out it needed to be reinforced and required UV protection.