Qouted from wikipedia:
"hardware-assisted virtualization is a platform virtualization approach that enables efficient full virtualization using help from hardware capabilities, primarily from the host processors".
Hardware-assisted virtualization reduces the maintenance overhead of paravirtualization as it reduces (ideally, eliminates) the changes needed in the guest operating system.
Hardware-assisted virtualization requires explicit support in the host CPU, which is not available on all x86/x86_64 processors.
Plenty of articles on the net, please do some basic research and please read FAQ.