Catalyzes the first step in biosynthesis of glycosaminoglycan. Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein. Initial enzyme in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans in fibroblasts and chondrocytes. L-seryl-[protein] + UDP-alpha-D-xylose = 3-O-(beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Glycan metabolism; chondroitin sulfate biosynthesis. Glycan metabolism; heparan sulfate biosynthesis. Belongs to the glycosyltransferase 14 family. XylT subfamily.