Allah(swt) sent many messengers to Mankind as Guides and Warners, some of them were given revelations while some were not. Revelations were sent through various methods and stages including through angels, inspirations, visions and directly, a special privilege of Musa (as) whom Allah(swt) spoke to directly. Reading The Sealed Nectar, I found the following as stated from Ibn Al-Qayyim.
Ibn Al-Qayyim, mentioning the stages of the Revelation, said:
The First: The period of true vision. It was the starting point of the Revelation to the Men of Allâh (Peace be upon him).
The Second: What the angel invisibly cast in the Prophet’s mind and heart. The Messenger of Allah (Peace be upon him) said: “The Noble Spirit revealed to me ‘No soul will perish until it exhausts its due course, so fear Allâh and gently request Him. Never get so impatient to the verge of disobedience of Allâh. What Allâh has can never be acquired but through obedience to Him.”
The Third: The angel used to visit the Messenger of Allâh (Peace be upon him) in the form of a human being and would speak to him directly. This would enable him to fully understand what the angel said. The angel was sometimes seen in this form by the Prophet’s Companions.
The Fourth: The angel came to him like the toll of a bell and this was the most difficult form because the angel used to seize him tightly and sweat would stream from his forehead even on the coldest day. If the Prophet (Peace be upon him) was on his camel, the camel would not withstand the weight, so it would immediately kneel down on the ground. Once the Messenger of Allah (Peace be upon him) had such a revelation when he was sitting and his thigh was on Zaid’s, Zaid felt the pressure had almost injured his thigh.
The Fifth: The Prophet (Peace be upon him) saw the angel in his actual form. The angel would reveal to him what Allâh had ordered him to reveal. This, as mentioned in (Qur’ân), in Sûrah An-Najm (Chapter 53 – The Star), happened twice.
The Sixth: What Allâh Himself revealed to him in heaven i.e. when he ascended to heaven and received Allâh’s behest of Salât (prayer).
The Seventh: Allâh’s Words to His Messenger (Peace be upon him) at first hand without the mediation of an angel. It was a privilege granted to Moses (Peace be upon him) and clearly attested in the Qur’ân, as it is attested to our Prophet (Peace be upon him) in the Sûrah Al-Isrâ’ (Chapter 17 – The Journey by Night) of the Noble Qur’ân.
Verily, We have sent the revelation to you (O Muhammad صلى الله عليه وسلم) as We sent the revelation to Nuh (Noah) and the Prophets after him; We (also) sent the revelation to Ibrahim (Abraham), Isma’il (Ishmael), Ishaq (Isaac), Ya’qub (Jacob), and Al-Asbat [the offspring of the twelve sons of Ya’qub (Jacob)], ‘Îsa (Jesus), Ayyub (Job), Yunus (Jonah), Harun (Aaron), and Sulaiman (Solomon); and to Dawud (David) We gave the Zabur (Psalms).
And Messengers We have mentioned to you before, and Messengers We have not mentioned to you, – and to Musa (Moses) Allah spoke directly.
Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allah after the (coming of) Messengers. And Allah is Ever All-Powerful, All-Wise.
But Allah bears witness to that which He has sent down (the Qur’an) unto you (O Muhammad صلى الله عليه وسلم); He has sent it down with His Knowledge, and the angels bear witness. And Allah is All-Sufficient as a Witness.