What is the difference between I'tikaf and Khalwa in Ramadan?

"Legitimate I'tikaf (spiritual retreat) is the staying in a mosque with the intention of worshipping Allah. It is sanctioned by the Qur'an, the Sunnah, and scholarly consensus. It is a Sunnah (recommended practice) during the last ten days of Ramadan, and it was prescribed for the purpose of reforming and purifying the soul, and severing its attachments to creation. As for the term "khalwa" (solitary retreat), scholars have not used it in the meaning of I'tikaf. It is more specific than I'tikaf in one sense, and more general in another. It is desirable if it is for remembrance (dhikr) and worship, and free from innovation (bid'ah)."