Vmware but very few hosting companies use it as it cost money for licenses compare to Xen & Openvz. You would pay a lot for hosting plans like that. Plus VPS are not just limited to Linux. You have windows as well for those ASP.Net sites.
Linux VPS and Windows VPS are two types of VPS hosting services. OpenVZ, XEN, KAM and VMWare are virtualization technologies you can get with these VPS services. Most of the VPS are now XEN virtualization based. It supports both Linux and Windows OS. XEN VPS are good choice for better performance.
There are many different types of VPS servers. Most providers offer two types of vps servers openvz and xen but there are a few more different types of servers. Every different type of vps server is built for certain requirements. Forexaple you may go with xen if you want nore power out of your resources instead of going with openvz for openvpn software.
All softwares are compatible and non compatible with different servers.
I dont know the exact number of different vps are out there but there must be a lot even some that we may of never heard of and are used for private use or even businesses that create their own stuff.