As per the Shiva Purana, there is a main God Shiva who is Nirakara and thereafter his physical manifestation who is known as Shankara. I know that different Hindu texts about different Gods treat their God as the Supreme nirakaar God, e.g. Mahavishnu, Parabramha, Devi Shakti, etc who created the other Gods and started the universe. I wanted to know whether such story exists in the case of Lord Vishnu as well that there is a Mahavishnu and then there is (are) Vishnu(s). I think such story exists but I have not read such Lord Vishnu-related texts. Can anybody please let me know if there is such extract anywhere in Hindu texts which tells the existence of both MahaVishnu and Vishnu? Also, please don't confuse Vishnu with his later incarnations such as Lord Rama or Lord Krishna.
First of all, yes, there is a notion of a greater Vishnu and a lesser Vishnu. This concept is expressed in simplest terms in this chapter of the Vishnu Purana:
Affecting then the quality of activity, Hari, the lord of all, himself becoming Brahmá, engaged in the creation of the universe. Vishńu with the quality of goodness, and of immeasurable power, preserves created things through successive ages, until the close of the period termed a Kalpa; when the same mighty deity, Janárddana, invested with the quality of darkness, assumes the awful form of Rudra, and swallows up the universe. Having thus devoured all things, and converted the world into one vast ocean, the Supreme reposes upon his mighty serpent couch amidst the deep: he awakes after a season, and again, as Brahmá, becomes the author of creation. Thus the one only god, Janárddana, takes the designation of Brahmá, Vishńu, and Śiva, accordingly as he creates, preserves, or destroys.
So we have a supreme Vishnu who reposes upon a serpent couch, and he manifests himself as a lesser Vishnu, the god of preservation (and as Brahma and Shiva as well).
But for more precision on all this we must turn to Pancharatra philosophy, which lies at the heart of mainstream Vaishnavism. As I discuss in this answer, the Pancharatra movement began with the sage Narayana, an ancient incarnation of Vishnu who was the son of Yama god of death and twin brother of the sage Nara. Narayana is said to have "become all things" after doing a five-day (Pancharatra) Yagna, so people started following Pancharatra texts to worship him. Since the Pancharatra texts originated with Narayana, they are fundamental to pretty much all mainstream Vaishnava sects. Among the oldest Pancharatra texts are the Satvata Samhita, the Jayakhya Samhita, and the Ahirbudhyna Samhita.
But there's a Pancharatra text that's even older than these and it's found in the Mahabharata! It's called the Narayaniya, and it's an 18-chapter religious discourse between Yudhisthira and Bhishma found in the Shanti Parva of the Mahabharata, similar to the Bhagavad Gita, another 18-chapter discourse between Arjuna and Krishna found in the Bhishma Parva. In any case, here is what Vishnu tells Narada in the Narayaniya portion of the Shanti Parva:
Assuming a form that is manifest, I dwell, at present, in the heavens. At the end of a thousand Yugas I shall once more with-draw the universe into myself. Having withdrawn all creatures, mobile and immobile into myself, I shall exist all alone with knowledge only for my companion. After the lapse of ages I shall again create the universe, with the aid of that knowledge. That which is my fourth form creates the indestructible Sesha. That Sesha is called by the name of Sankarshana. Sankarshana creates Pradyumna. From Pradyumna I take birth myself as Aniruddha. I create (myself) repeatedly. From Aniruddha springs Brahma. The latter takes birth from Aniruddha's navel. From Brahma spring all creatures mobile and immobile. Know that Creation springs in this way repeatedly at the beginning of every Kalpa.
This is a good delineation of the various forms of Vishnu (see this Wikipedia article for more details). First of all, there is the supreme Vishnu known as Para Vasudeva, who dwells in Vishnu's highest abode of Paramapadam. From him manifests the four Vyuha forms of Vishnu: Vyuha Vasudeva, who dwells in the ocean of milk and is visible to the gods; Sankarshana, from whom emerges Vishnu's serpent Adiseshan and who is ultimately responsible for the destruction of the Universe (the Srimad Bhagavatam calls him the heart of Shiva); Pradyumna, who is responsible for the day-to-day preservation of the Universe; and Aniruddha (whom I discuss here), the thousand-armed form of Vishnu from whose navel came the lotus flower containing Brahma. Note that it's not a coincidence that these are the names of Krishna, his brother Balarama, his son Pradyumna, and his grandson Aniruddha; all four Vyuha forms incarnated in the Yadava race.
The Pancharatra distinction I described above are the ones universally accepted across the different sects of Vaishnavism, but I should mention that the Gaudiya Vaishnava sect, which includes the popular organization ISKCON, makes further distinctions. This is based on a Gaudiya Vaishnava text called the Satvata Tantra, which says this:
Vishnu has three forms called purusas. The first, Maha-Vishnu, is the creator of the total material energy (mahat), the second is Garbhodasayi, who is situated within each universe, and the third is Ksirodasyi, who lives in the heart of every living being.
So the idea is that you have countless Brahmandas or Universe, each ruled by its own Garbhodasayi Vishnu, and all these Vishnus are manifested from one being called Mahavishnu (whom Gaudiya Vaishnavas believe is himself an expansion of Krishna).
By the way, you also asked whether Vishnu has a formless aspect. Yes, he does; in the chapter of the Shanti Parva I linked to above, Vishnu also tells Narada this:
I am known as Purusha. Without acts, I am the Twenty-fifth. Transcending attributes, I am entire and indivisible. I am above all pairs of opposite attributes and freed from all attachments. This, O Narada, thou wilt fail to understand. Thou beholdest me as endued with a form. In a moment, if the wish arises, I can dissolve this form. I am the Supreme Lord and the Preceptor of the universe. That which thou beholdest of me, O Narada, is only an illusion of mine. I now seem to be endued with the attributes of all created things. Thou art not competent to know me. I have disclosed to thee duly my quadruple form. I am, O Narada, the Doer, I am Cause, and I am Effect. I am the sum-total of all living creatures. All living creatures have their refuge in me. Let not the thought be thine that thou hast seen the Kshetrajna. I pervade all things. O Brahmana, and am the Jiva-Soul of all creatures. When the bodies of all creatures, however, are destroyed, I am not destroyed.