Question: **I imagine that no, they aren't, because every process has its own memory space, of course.**

But how does the whole JVM thing actually work? Is there a separate JVM in a separate process for every Java program that I launch? Do Java programs running in a system share anything at all? Are there differences between OSs and JVM implementations? Can I *make* programs share variables (i. e. directly through the JVM rather than the usual IPC mechanisms)? Are there more exotic one-process JVMs for special purposes?

Generally, what are recommendable reads about the guts of JVMs? The [spec](http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html)? The source code of some implementation? Websites? Books?

Answer: 
you should probably search more in the area of class loading if your main interest is about how static class members are instantiated. 

Different threads on the same VM may have their own static class nember instances if they used separate class loaders. 

The classic example here would be [Apache Tomcat](http://tomcat.apache.org/) keeping different web applications separate, though lib jars might be common to all applications.
Just to follow on to what Kevin mentioned, there are no JVM implementations that I am aware of that allow you to share static variables across JVM instances. As previous answers have stated (correctly) - each JVM instance is it's own process.

Terracotta (which is a clustering technology, not a JVM) allows for arbitrary sharing of object instances across JVM process boundaries - including sharing static variables. Thus the moniker "network attached memory". For all intents and purposes, when using Terracotta technology in your JVM process, all of the threads in all of the VMs behave as if they were all looking at a single, large, shared heap. (With caveats of course, since that's not physically possible, Terracotta manages it through sophisticated sharing and replication algorithms which involve network io to move data - so therefore there are times when latency and throughput will not compare to native memory access)

There are abundant examples in the Cookbook section of the Terracotta site - I recommend you start with the Hello [Clustered Instance Recipe](http://www.terracotta.org/web/display/orgsite/Recipe?recipe=helloclusteredinstance) to get a feel for how it works.