Recently I was comparing servlet specifications after 2.4 version, Here is a short matrix for reference

• Servlet 3.1 (JSR 340)
• Platform: JavaEE 7
• Compatible Application Server & Servlet Containers:
  • http://tomcat.apache.org/whichversion.html
  • http://www.oracle.com/technetwork/java/javaee/overview/compatibility-jsp-136984.html
• New Features:
  • Non Blocking IO
  • HTTP protocall Upgrade
  • Security Enhacements
• Release Date: April 2013

• Servlet 3.0 (JSR 315)
• Platform: JavaEE 6, JavaSE 6
• Compatible Application Server & Servlet Containers:
  • http://tomcat.apache.org/whichversion.html
  • http://www.oracle.com/technetwork/java/javaee/overview/compatibility-jsp-136984.html
• New Features:
  • Web Fragements
  • ServletFilter Annotations
  • ServletContextListener Annotations
  • Filter Annotations
  • Async Processing for Servlets
  • Security Enhacements
• Release Date: Dec 2009

• Servlet 2.5 (JSR 154)
• Platform: JavaEE 5, JavaSE 5
• Compatible Application Server & Servlet Containers:
  • http://tomcat.apache.org/whichversion.html
  • http://www.oracle.com/technetwork/java/javaee/overview/compatibility-javaee5-jsp-141098.html
• New Features:
  • JavaSE5 requirement
  • Annotations
  • web.xml Changes
  • Boundary Conditions Clarifications
• Release Date: Nov 2003

Here are some great articles for further reading to get details list of features in each release.

• http://www.javaworld.com/article/2071393/java-web-development/new-features-added-to-servlet-2-5.html
• http://java.dzone.com/articles/an-overview-servlet-30
• https://blogs.oracle.com/arungupta/entry/what_s_new_in_servlet