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. Required for vulval morphogenesis and zygotic cytokinesis, suggesting that glycosaminoglycans play a central role in vulval morphogenesis. 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. Worms are infertile because of the failure of the progeny of homozygous mutants to initiate cytokinesis and because of the failure to form an extracellular space between the egg and the eggshell. Belongs to the glycosyltransferase 14 family. XylT subfamily.