The Absolute Truth, Krishna, is anandamaya (desiring to increase His joy), hence from His original form, He expands and becomes many. The Lord's various expansions and the Lord Himself simultaneously co-exist. These expansions of the Supreme Being are known as Vishnu-tattva, and they are also Supreme Absolute Truth. Although non-different from the Supreme Lord, the Vishnu-tattvas all accept their own role as Godhead in the mood of Servitor to the "original" Supreme Being. This relationship is the foundation of Vedic monotheism, which encompasses the inconceivable, absolute personal nature of the original Godhead.

The Supreme Being possesses two other expansive qualities known as Shiva-tattva and Jiva-tattva. Shiva-tattva is between Vishnu-tattva and jiva-tattva.

A jiva (spirit souls like us) can never attain the position of Shiva Tattva what to speak of Vishnu Tattva. Therefore all the myth propagating that man is God in illusion, daridra-narayan, are all misleading.

Krishna, the Original and Supreme Personality of Godhead possesses 100% of the qualities, Vishnu posses 94% of His qualities, Shiva 84% and Jiva only 78%. Therefore even after meditation for millions of lifetimes, a Jiva can never become Shiva or Vishnu or Krishna.

Shiva is neither an ordinary living entity, jiva, nor a typical demigod. He is in his own category of existence, known as shiva-tattva. He is almost on the level of Vishnu, but because of his interaction with material nature, he is not counted as Vishnu-tattva, the category of Godhead.

When Krishna expands into a form, whose purpose is to assist Krishna in annihilation, this form is called Shiva. An example cited in scriptures is like Milk is Transformed into Curd by adding acid, Krishna expands Himself into Shiva by a mixture of Ignorance. Also although curd is made out of milk, it cannot be converted back to milk, similarly, although Krishna expands as Shiva, Shiva can not be Krishna.

kṣīraḿ yathā dadhi vikāra-viśeṣa-yogāt
sañjāyate na hi tataḥ pṛthag asti hetoḥ
yaḥ śambhutām api tathā samupaiti kāryād
govindam ādi-puruṣaḿ tam ahaḿ bhajāmi

Just as milk is transformed into curd by the action of acids, but yet the effect curd is neither same as, nor different from, its cause, viz., milk, so I adore the primeval Lord Govinda of whom the state of Śambhu is a transformation for the performance of the work of destruction.
[Brahma Samhita 5.45]

Factually Krishna is the origin of everything existing including Shiva. This has been confirmed in Bhagavad Gita as well.

ahaṁ sarvasya prabhavo
mattaḥ sarvaṁ pravartate
iti matvā bhajante māṁ
budhā bhāva-samanvitāḥ

I am the source of all spiritual and material worlds. Everything emanates from Me. The wise who perfectly know this engage in My devotional service and worship Me with all their hearts.
[Bhagavad Gita 10.8]

ahaṁ kṛtsnasya jagataḥ
prabhavaḥ pralayas tathā
... Of all that is material and all that is spiritual in this world, know for certain that I am both the origin and the dissolution.
[Bhagavad Gita 7.6]

In the following verses, Lord Chaitanya Mahaprabhu very clearly describes Shiva while explaining the science of Absolute Truth to Sanatana Goswami

Lord Kṛṣṇa, the Supreme Personality of Godhead, expands a portion of His plenary portion and, accepting the association of the material mode of ignorance, assumes the form of Rudra to dissolve the cosmic manifestation. Rudra, Lord Śiva, has various forms, which are transformations brought about by association with māyā. Although Rudra is not on a level with the jīva-tattvas, he still cannot be considered a personal expansion of Lord Kṛṣṇa. Milk is transformed into yogurt when it associates with a yogurt culture. Thus yogurt is nothing but milk, but still it is not milk. ‘Milk changes into yogurt when mixed with a yogurt culture, but actually it is constitutionally nothing but milk. Similarly, Govinda, the Supreme Personality of Godhead, assumes the form of Lord Śiva [Śambhu] for the special purpose of material transactions. I offer my obeisances at His lotus feet.’ Lord Śiva is an associate of the external energy; therefore he is absorbed in the material quality of darkness. Lord Viṣṇu is transcendental to māyā and the qualities of māyā. Therefore He is the Supreme Personality of Godhead. ‘The truth about Lord Śiva is that he is always covered with three material coverings — vaikārika, taijasa and tāmasa. Because of these three modes of material nature, he always associates with the external energy and egotism itself.’ ‘Śrī Hari, the Supreme Personality of Godhead, is situated beyond the range of material nature; therefore He is the supreme transcendental person. He can see everything inside and outside; therefore He is the supreme overseer of all living entities. If someone takes shelter at His lotus feet and worships Him, he also attains a transcendental position.’ For the maintenance of the universe, Lord Kṛṣṇa descends as His personal plenary expansion in the form of Viṣṇu. He is the director of the mode of goodness; therefore He is transcendental to the material energy. Lord Viṣṇu is in the category of svāṁśa because He has opulences almost equal to Kṛṣṇa’s. Kṛṣṇa is the original person, and Lord Viṣṇu is His personal expansion. This is the verdict of all Vedic literatures. ‘When the flame of one candle is expanded to another candle and placed in a different position, it burns separately, and its illumination is as powerful as the original candle’s. Similarly, the Supreme Personality of Godhead, Govinda, expands Himself in different forms as Viṣṇu, who is equally luminous, powerful and opulent. Let me worship that Supreme Personality of Godhead, Govinda.’ The conclusion is that Lord Brahmā and Lord Śiva are simply devotee incarnations who carry out orders. However, Lord Viṣṇu, the maintainer, is the personal feature of Lord Kṛṣṇa. [Lord Brahmā said:] ‘I am engaged by the Supreme Personality of Godhead to create. Following His orders, Lord Śiva dissolves everything. The Supreme Personality of Godhead in His form of Kṣīrodakaśāyī Viṣṇu maintains all the affairs of material nature. Thus the supreme controller of the three modes of material nature is Lord Viṣṇu.’
[ Śrī Caitanya-caritāmṛta Madhya-līlā 507 - 518]