KVM is full hardware virtualisation, you can run almost any operating system as a guest BSD/Windows/Linux and with virtio driver you will get near native performance, some experiments have shown only 3% loss on native hardware under ideal circumstances.
It does support installation from ISO and also template based installation, it comes with good separation in terms of privacy, it can suffer I/O lag under heavy load which impacts the guest operating systems and the host operating system.
Each guest (VM) runs as a process on the host node while this is great for discovering which guest is causing issues when required it can also cause problems if the host is under heavy load all guests suffer.
You can over allocate Ram with little effort however in most cases unless the host is full SSD this is unlikely to happen due to the overhead it puts on the host node and subsequent and obvious performance issues.
Due to KVM being native in most modern kernels it does lend a performance advantage over others in some circumstances and is still fairly new and under very active development.
Most people select KVM for excellent performance and flexibility although perhaps not quite as stable as Xen due to maturity.