The HaLVM allows you to run Haskell programs on bare (virtual) metal - inside a Xen domain, without a host operating system.
To put it another way: the HaLVM allows you to run a web server, or key manager, or a pod of virtual machines comprising a IPSec network stack, or anything you can write in Haskell - without an operating system weighing you down. Because a HaLVM has a small footprint, it requires fewer resources and boots in milliseconds. And you can fit a lot more on them on your Xen host.
Not only do HaLVMs boot faster than most virtual machines, they are also arguably safer. For one, they do not suffer from the complexity and inherent bugginess of the operating system layer and all of its associated vulnerabilities. And, most of the HaLVM is written in Haskell, a higher-order, pure functional language in which it is literally impossible to express many common bugs (such as NULL pointer dereference, or buffer overflow).