The hardware version of a virtual machine indicates the lower-level virtual hardware features a virtual machine supports, such as BIOS, number of virtual slots, maximum number of CPUs, maximum memory configuration, and other hardware characteristics.
For a newly created virtual machine, the default hardware version is the most recent version available on the host where the virtual machine is created. To increase compatibility, you might want to create a virtual machine with a hardware version older than the highest supported version. You can do so by specifying the VirtualMachineConfigSpec.version property during virtual machine creation. For existing virtual machines, call the VirtualMachine.UpgradeVM_Task method.
The hardware version of a virtual machine can be lower than the highest version supported by the ESXi host it is running on under the following conditions:
- You migrate a virtual machine to a newer version of ESXi after it was created on a host that was running an earlier version of ESXi.
- You create a virtual machine on a newer version of ESXi by using an existing virtual disk that was created on a host that was running an earlier version of ESXi.
- You add a virtual disk created on a host that was running an earlier version of ESXi to a virtual machine created on a newer version of ESXi.
Virtual machines with hardware versions lower than 4 can run on ESX/ESXi 4.x hosts but have reduced performance and capabilities. In particular, you cannot add or remove virtual devices on virtual machines with hardware versions lower than 4 when they reside on an ESX/ESXi 4.x host. To make full use of these virtual machines, upgrade the virtual hardware.
