The literal meaning of effervescent is bubbly or sparkling, like sparkling wine (wine with little bubbles in it, often called Champagne). In English, we also use it to describe personalities -- especially, a personality that is outgoing, happy, and enthusiastic. Ebullient usually is not used to describe personalities, but instead to describe a feeling or emotion -- an emotion of great happiness or joy. Ebullient is something you may be for a short time when something really good happens, then later you return to normal. But if you have an effervescent personality, that is a permanent trait of personality, not a short-time feeling.