primary xylem is primary in nature & is derived from procambium. But secondary xylem is secondary in nature and derived from fascicular cambium and interfascicular cambium. Primary xylem is differentiated into protoxylem and metaxylem, but secondary xylem has no such differentiation. In primary xylem vessels and tracheids are long and narrow, and vessels don't have tyloses, but in secondary xylem, vessels are blocked by tyloses, and vessels and tracheids are wider and shorter. Xylem fibres are more abundant in secondary xylem , and found in small numbers in primary xylem. Also unlike in primary xylem, secondary xylem has differentiated into sapwood & heartwood.

Secondary xylem

Xylem

secondary xylem