The paravirtualisation settings is controlled by the line vmi.present in the VMX file of the guest.
See Carter's blog entry on how to change the VMX file from within PowerShell.
The following script shows how enable paravirtualisation.
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.extraConfig += New-Object VMware.Vim.OptionValue
$spec.extraConfig[0].key = "vmi.present"
$spec.extraConfig[0].value = "true"
(get-view (Get-VM -Name <VM-name>).ID).ReconfigVM($spec)