Catalyzes the first step in the biosynthesis of chondroitin sulfate and dermatan sulfate proteoglycans, such as DCN. Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein. Required for normal maturation of chondrocytes during bone development, normal onset of ossification and normal embryonic and postnatal skeleton development, especially of the long bones. 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. Monomer. Contains 7 disulfide bonds. N-glycosylated. Belongs to the glycosyltransferase 14 family. XylT subfamily.