JVM is an acronym for Java Virtual Machine. The JVM contains the Java "CPU" - which runs Java Byte Code instructions - and the supportive libraries.
The iSeries supports multiple full featured Java Virtual Machines, all invoked through the same JAVA/RUNJVA command. The system property "java.version" tells JAVA which JVM to start.
The traditional JVM is 64-bit which gives a very large heap space but also uses up a lot of memory. From 5.4 there is a new 32-bit JVM which uses only about half of the memory needed by the 64-bit JVM for a given task. As it is ported from AIX it requires the PASE environment.
Java 1.5 is avaliable from V5R1, and Java 6 is available from V5R4.