OpenVZ
Look for /proc/vz/veinfo
Xen
Look for one of /proc/sys/xen, /sys/bus/xen or /proc/xen
vserver
Look for s_context or VxID in /proc/self/status
VMWare or Parallels
- Run lspci and look for VMWare VGA adapter
- Run dmidecode and look for mention of VMWare or Parallels
KVM
Run lspci and look for RAM memory: Qumranet, Inc. Virtio memory balloon