I expected this to be about system virtualization, not language VMs. Interesting though, but not my area of expertise.
The blatant reuse of abbreviations and terms is one of my pet peeves.
For example, VM can mean Virtual Machine, Virtual Machine, or Virtual Memory. Yep, three completely distinct things, two of which are helpfully referred to under the same expanded name as well. Do you know all three? mroche has helpfully disambiguated between Virtual Machine and Virtual Machine already, but in case you need help:
Virtual Machine means a computer split up into many different fake systems, each running a guest OS. Xen is an example of a Virtual Machine hypervisor.
Virtual Machine means a completely fake CPU packaged with libraries and used to run a high-level language, to facilitate things like garbage collection and language-level security. Java runs on a Virtual Machine, quite imaginatively called the Java Virtual Machine. Now, what would we call a Java execution environment which could run as a Xen guest? (Don't strain yourself... )
Virtual Memory means lying to applications about how memory works, to present a completely flat address space without such annoyances as caching or other programs or the operating system disturbing application programmers, who are quite disturbed enough.
Now, let's take all of those concepts, refer to all three of them with the same abbreviation and two of them with the same name entirely. I suppose I should feel lucky to live in this time: Soon, we'll be calling them all Bruce, to cut down on confusion.
Sure, xen simulates the same CPU type as host, and talks to host via block devices and raw sockets, and each guest includes the full OS and filesystem; while Java VM simulates a completely different CPU, and uses host's OS for filesystem and TCP/IP access. They seem pretty distinct.
But you are just looking at the sides of the spectrum, and there are plenty of things in the middle.
- With Xen, you can boot directly into user app, without any OS, filesystems or separate libraries. Is it still "fake system" if the thing you are booting has no chance of running on a real hardware?
- Qemu/kvm, which is normally used to emulate processors, supports "virt", "a platform which doesn't correspond to any real hardware and is designed for use in virtual machines." Does this start to sound like JVM for you?
- There are an actual, physical chips which execute Java bytecode directly (like picoJava). Does this put Java VM into "hardware emulation" category?
- On, and there is UML (user mode linux) project -- it emulates a virtual machine with its own fixed memory pool (like xen), and can use host's block device (like xen), and raw networking (like xen); but it can also use host's filesystem (like JVM), and it uses host's kernel for thread scheduling (like JVM). Where does it go?
- Oh, and there is a Smalltalk. The older versions had garbage-collecting VM (like JVM), but it had its own device drivers (like xen) and filesystem (like xen).
There is a reason we call of them "virtual machines" -- they have lots of things in common.
At one point I interacted with the JVM and its quirks on a daily basis, and I never started referring to it as "the VM", it was always "the JVM".
I suspect in large part because they're not that different from a systems perspective, I assume "VM" to mean the more generic of the two!
In more general contexts I tend to write "language VMs" explicitly.
Perhaps the most striking frequent differences between a typical OS VM and a typical language VM is memory management.
...Which is the very thing that the author sets out to bridge the gap for. Were higher-lever memory management available in the OS, the line would become truly fuzzy between the two.