Doing the below should work:
$details = get-vm | Select Name,@{N="Configured OS";E={$_.ExtensionData.Config.GuestFullname}},@{N="Running OS";E={$_.Guest.OsFullName}}, @{N="Powered On";E={ $_.PowerState -eq “PoweredOn”}}, @{N="disktype";E={(Get-Harddisk $_).Storageformat}}
$details | Export-CSV -NoTypeInformation C:\Users\iaasadmin\Desktop\getvms.csv
Or you can club both the statements together by piping the output to Export-CSV like below:
get-vm | Select Name,@{N="Configured OS";E={$_.ExtensionData.Config.GuestFullname}},@{N="Running OS";E={$_.Guest.OsFullName}}, @{N="Powered On";E={ $_.PowerState -eq “PoweredOn”}}, @{N="disktype";E={(Get-Harddisk $_).Storageformat}} | Export-CSV -NoTypeInformation C:\Users\iaasadmin\Desktop\getvms.csv