Who modifies the source code of guest OS in para virtualization?

Para virtualization is a virtualization technique that presents a software interface to virtual machines that is similar, but not identical to that of the underlying hardware. Para virtualization requires the guest operating system to be explicitly ported for the para-API , hence modifications are required and sometimes inevitable.  The source code of the guest OS may be provided by the developer itself , in this case the implementing team can modify. Sometimes ,when the operating system cannot be modified, components may still be available that can enable many of the significant performance advantages of para virtualization.