XML Encryption encompasses the encryption of any kind of data, including the encryption of XML. What makes it XML Encryption is that an XML element (either an EncryptedData or EncryptedKey element) contains or refers to the cipher text, keying information, and algorithms.
Both XML Signature and XML Encryption use KeyInfo element. It appears as the child of a SignedInfo, EncryptedData, or EncryptedKey element and provides information to a recipient about what keying material to use in validating a signature or decrypting encrypted data.
KeyInfo is optional: it can be attached in the message, or be delivered through a secure channel