I come across Terracotta as a distributed JVM that is stable and robust, however as my understanding of Terracotta is shallow, i am not sure if Terracotta is an alternative to JVMs?
It looks promising.
Terracotta is not a JVM, it's a shared memory backplane. It does not replace the JVM in any way. Terracotta now owns and makes several projects which can leverage this backplane.
They include
Amongst others. Their product BigMemory also helps to overcome some of the JVM Heap issues with very large allocations.