A hypervisor is a layer of software that creates one or more virtual hardware systems. This allows multiple system images (operating system instances) to run at the same time (Wikipedia). Understanding how the hypervisor is implemented will let you predict the relative performance overhead compared to other forms of virtualization. It will also help you understand the features and limitations of each form. The table below sorts the different forms of virtualization from the more concrete (closer to the hardware) to the more abstract (further away from the hardware).

 

Product Names Location of HV Explanation
SunFire / M-Series
Dynamic System Domains
aka Hard Partitions 		none HW routing to machine partitions
is done using HW only.
LDOMS aka
Oracle VM Server for SPARC 		firmware
Xen / Sun xVM aka
Oracle VM Server for x86 		kernel replacement aka Type 1
Linux KVM kernel integrated
Virtualbox / VMWare application aka Type 2
Solaris Zones aka Containers none HW resources are virtualized from the application
viewpoint, not from the kernel viewpoint.

Note: This is an update to a post two years ago.  This one supersedes the older post.