There are different JVM implementations available apart from popular Sun's (now Oracle) hotspot JVM like IBM's JVM. Now the question comes, what makes two JVM different from each other? Quite a lot of things can be different even if two JVMs are implemented by following Java virtual machine specifications. IBM has its own JVM implementation which is different than Oracle's JVM in many things like JIT(Just in Time) Compiler, the structure of Java heap, garbage collectors, and support for monitoring and management. Though I have mostly used Sun’s hotspot JVM recently got a chance to run some Java programs on IBM JVM on the AIX platform.