Vritual Machine on x86

OS architecture design and implementation comes from
simple structure to classic layered Unix system approach
which Lunix and Windows follows. Furthermore, to simplify
the kernel manageability, Micro kernel architecture was
proposed. However due to performance and scalability,
Solaris modulization design won the game. One interesting
thing is that Mac OS X takes hybrid structure which bridge
the layered BSD kernel design with Microkernel implementation.
Microkernel manages memory, RPC, IPC and Kthread scheduling.
BSD kernel does the CLIs, file systems and all the POSIX APIs.

Traditional layered Solaris Kernel design concludes the concept
of abstracting the HW resource into several execution environments.
With such virtualization techniques, a process is provided with a
virtual copy of underline OS and HW resources.

Therefore the fundamental resource to run virtual machine is to
share the HW with different execution environments. In such way,
Virtual machine is running in the kernel mode and execute at the
user mode. It has relative virtual user and kernel modes. If there is
a process running in a virtual machine, the control will be transfered
from virtual machine monitor to change the register and process program
counter for simulating the system call. Hence the major difference is
the real I/O will take much more time than virtual I/O does. CPU instruction
time will increase due to the multi-processes running within each virtual
machine. Virtual machine model is the best fit for R&D

However, it seems virtual machine can help resolve system compatibility
issues. The two popular favors of the virtual machine are: vmware and
Java VM. Since virtual machines are running on the top of OS, the traditional
OS design and implementation such Solaris Modules, Microkernel, VM are
still applied.

VMware abstracts x86 platform into isolated virtual machines. VMware runs
as user land application on the top of host OS which enable multiple guest
OSs concurrently within each virtual machines. However, the virtualization
layer as the core the vmware is the most expensive design to abstract the
underline resources into various virtual machines as guest OSs. Each vm
has it's own CPU, Memory, devices etc.

JVM is also abtracting the underline OS and HW. It is through class loader
and Java interpreters to execute the byte codes.

In general, the question of the design and utilization of virtual machine
is depends on the level of virtualization which fits in the requirements.
For platform and system level virtualization across different guest
OSs, vmware is the choice. However, if you only want to virtualize the
user land applications specifclly for Java applications, JVM is the right
technical and political answers to acorss different OSs. An important
note, application level virtualization has been done significantly by Sun
ISVs such as Cassat for Java EE virtualization.