Con Considerable overhead
Depending on the host hypervisor may take up anywhere from 3% of the hosts memory and cpu resources up to 35%.
Low density, performance, scalability. While some issues are mitigated with new hardware features such as VT-D, the overhead is considerable.
In certain specialized cases many of these issues can be sidestepped via the use of unikernels. Unikernels are specialized, lightweight operating systems for hypervisors. They are built to run only the necessary processes with only the necessary parts of necessary libraries in order to vastly increase performance. They reduce the memory footprint, the need for disk space and computational burden. Additionally they further reduce the attack surface.