Shadowfeed card has following text:
Exile target card from a graveyard. You gain 3 life.
Does it mean my graveyard, or any graveyard, including my opponents'?
The card doesn't specify which graveyard, so it can be any graveyard. That means that you can exile any card from your graveyard, one of your opponents' graveyards, or even one of your allies' graveyards in certain kinds of multiplayer games.
It means you pick the graveyard. So, it could be your graveyard or one belonging to an opponent; i.e. any graveyard. If it were supposed to only apply to your or an opponent's graveyard, the card would say so.