Catalyzes the first step in the biosynthesis of chondroitin sulfate, heparan sulfate and dermatan sulfate proteoglycans, such as DCN (PubMed:17517600). Transfers D-xylose from UDP-D-xylose to specific serine residues of the core protein (By similarity). L-seryl-[protein] + UDP-alpha-D-xylose = 3-O-(beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Active with either Mg(2+) or Mn(2+), but activity is highest when both are present. Glycan metabolism; chondroitin sulfate biosynthesis. Glycan metabolism; heparan sulfate biosynthesis. Monomer. Detected in brain, liver, lung, kidney, heart, spleen and testis, and at lower levels in skeletal muscle. Contains disulfide bonds. Mutant mice are born at the expected Mendelian rate. Their livers display strongly reduced levels of heparan sulfate proteoglycan. DCN glycosylation is altered and lacks chondroitin sulfate groups. After 3 to 5 months, all mutant mice display increased liver weight. At an age of 4 to 5 months, about half of them delevop liver cysts, due to biliary epithelial cell hyperplasia. At an age of 3 and 10 months, mutant mice also display increased kidney weight due to hydronephrosis and impaired renal function, but they do not develop cysts. Belongs to the glycosyltransferase 14 family. XylT subfamily.