Virtual Appliances

These are software appliances (mostly enterprise applications) that run on a Virtual Machine. A virtual machine (VM) is a software implementation of a machine (i.e. a computer) that executes programs like a physical machine.

Virtual Machines are realized using a Virtualization Platform:
1. Sun Virtual Box (Now called Oracle VM Virtualbox)
2. VMWare
3. Parallels Workstation, etc

The main advantages of system VMs are:
i) multiple OS environments can co-exist on the same computer, in strong isolation from each other
ii) the virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine
iii) application provisioning, maintenance, high availability and disaster recovery[2]

The main disadvantage of system VMs is:
a virtual machine is less efficient than a real machine since it accesses the hardware indirectly


A virtual appliance is a virtual machine image designed to run on a virtualization platform.

Virtual appliances are a subset of the broader class of software appliances. Installation of a software appliance to a virtual machine creates a virtual appliance. Like software appliances, virtual appliances are aimed to eliminate the installation, configuration and maintenance costs associated with running complex stacks of software.

A virtual appliance is not a virtual machine, but rather a software image containing a software stack designed to run inside a virtual machine. Like a physical machine, a virtual machine is merely a platform for running an operating system environment and by itself does not contain application software.