You could say:
1. A. is part-government-owned.
It wouldn't necessarily be informal. Cf.
2. A. is part-owned by the government.
But #1 might seem a little ungainly.
"Partially" has a sense of "incompletely", so might suit certain contexts:
3. Nonsense! A. is only partially government-owned, not wholly government-owned!
For neutral usage, however, I would choose "partly".
All the best,