You may try below one, should work for filtering only "Windows Xp" guest VMs.

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$vms = Get-VM

Foreach ($vm in $vms){

     if ((Get-View $vm).Guest.GuestFullName -match "windows xp"){

               Write-Host $vm.Name, " => " , (Get-View $vm).Guest.GuestFullName

     }

}

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You can check keeping "Xp" instead of "Windows Xp", it should also work..

Thanks,

Amit