From http://pycsw.org/development/rfc/rfc-4.html :
CSW 3.0 provides major features and improvements over CSW 2.0.2 as part of the evolution of OGC Catalogue Services, including Open Search Geo and Time Extensions, OpenSearch 1.1, and the Atom Syndication Format. Other enhancements include:
- features advertised as conformance classes simpler KVP API
- enhanced distributed searching functionality
- raw metadata response for GetRecordById
- proper use of HTTP status codes
- proper use of HTTP request/response headers *UnHarvest operation
- use of temporal predicates for query and presentation
Hope this helps.
..Tom
P.S. there are out of the box CSW server implementations out there already like pycsw and GeoNetwork OpenSource which you may want to consider leveraging.