JBossWeb native is based on The Apache Tomcat Native. The packages available in JBossWeb Native a bundle of tc-native, OpenSSL, apr and additional libraries. They allow the easy addition of the JBossWeb extensions to JBossAS.
Apart high performance compared to JIO you need to install JBossWeb Native if you want to use Comet
Note that Comet is not (yet) in the Servlet Specification so the JBossWeb Comet implementation is not 100% compatible with the Tomcat one.