Dedicated/Private JVM (Java Virtual Machine)

Private JVM means exactly that - you are provided with a distinct JVM that is in your exclusive use. Other users do not have access to you JVM, and their bugs can not affect your web site. If your web application is a quality one, the Private JVM can be run for long periods of time, providing the best performance.

On contrary, some hosting companies offer Shared JVM. Such Shared JVM usually holds dozens (or hundreds) web sites, and if one of web application screws things up (consuming all JVM memory, for example) - all hosted web sites suffer. Shared JVMs tend to possess uncollectible garbage over time; for this reason, Shared JVM is restarted regularly which seriously affects performance and causes some of your web site visitors to see error messages when restart takes place.