vhd to ova conversion basically represents conversion between two "virtual hard drives" used in different virtualization programs. As far as we know, virtual drives in VHD format (.vhd) can be converted to Open Virtual Machine Format (.ova) with virtualization software such as VirtualBox, or with dedicated converters like XenConvert.
Open VHD file Open OVA fileFree system virtualization platform for Linux operating system developed by Oracle
A free virtual machine converter by Citrix