Instructions
Go to a Pokémon Center. Like all versions of the game, the quickest and most straightforward way to heal your party is to head to a Pokémon center. They appear in most towns and cities. Speak to the person at the desk, who will offer to restore all your Pokémon to full health.
Find a healing machine. As Pokémon XD is centered more on battling than other versions of the game, there are several healing machines dotted around. Many of these machines are located in areas where you are taking part in a lot of battles, which allows you to make regular stops.
Use a potion. Potions can be purchased at any Poké Mart, but are also dotted around various places in the game where they can be picked up for free. There are different kinds of potions with a range of strengths. A max potion will fully revive a Pokémon's health, providing it still has some left; while an ordinary potion will only restore it by 20 hit points. (ref 4)
Use a recovery move or HP draining attack. Some Pokémon can learn moves that restore health. Recovery moves such as rest, recover and slack off simply give a Pokémon some HP back. HP draining attacks, like Giga Drain and Dream Eater, cause damage to the defending Pokémon and transfers some of it to the attacker.
Buy drinks. There is a vending machine in Pyrite town that stocks several HP recovery drinks. These are better value for money than potions. The cheapest is fresh water, which costs 200 Poké Dollars for 50 HP, while Moomoo milk restores 100 HP for 500 dollars.