It is sometimes pointed out that the Brahman of Kashmir Shaivism (or Paramaśiva as it is referred to there) is not all that different from the Brahman of Advaita Vēdānta. And the inference, according to some observers, is that Parādvaita (of the Agamic Shaivism propagated by Mahāmāhēśvara Abhinavagupta) is not all that different from Kēvalādvaita (as propagated by ācārya Srī Shaṅkara Bhavatpāda) and reconciliation between the two is not only possible but easy enough to accomplish.
First of all, is reconciliation between the two even necessary’ We have been ourselves guilty in the past of some such attempts and have realized the futility and gross uselessness of such unnecessary intellectual gymnastics. As far as Parādvaita of Abhinavagupta is concerned, there are some key differences. For example, if one studies his īśvarapratyabhijñāvivr̥ti, he expresses severe criticism of the concept of avidyā and its acceptance as the source of creation. He refuses to accept the indescribability of a beginningless ignorance that can delude the finite beings, as also the concept of a non-substantial entity manifesting a drama as extraordinary as the creation.
His next attack is on the theory of Vivartavāda dear to Kēvalādvaitins: he questions how something can be unreal if it has manifested (the definition of Vivarta being the apparent manifestation of an unreal entity). So, basically, he postulates that something that clearly manifests and results in creation has to be both real and substantial and hence describable. In fact, he specifically states in his Parātrimśikāvivaraṇa that the Brahman of Advaita Vēdānta is not the same as that of Parādvaita, but is instead very similar to the ultimate nihilistic principle of the Buddhists (a charge leveled by others such as Yāmunācārya and Rāmānuja as well).
He further elaborates by criticizing both the Buddhists and Advaita Vēdāntins thus: ‘Both the Bauddhas and Vēdāntins were unable to satisfactorily explain unity and diversity and hence deceived themselves and the world by terming the apparent diversity as indescribable because of its being avidyā (this is the criticism against Advaita Vēdāntins) and by calling it false because it was simply due to mental ideation (as in the case of Buddhists)”. The very fact that Abhinavagupta coined the term ‘Parādvaita’ to distinguish his version of monism from Advaita Vēdānta ‘ illustrates the theological difference between the two schools.
At a practical level, Abhinavagupta points out that the monism of Vēdāntins can be experienced only in a state of samādhi or trance whereas in the case of Parādvaita, this unity is directly experienced in every worldly transaction as well. Hence, he qualifies his school with the word, pratyakṣādvaita. He also states clearly that Paramaśiva of Parādvaita is not some form of personal God or Saguṇa Brahman (using the terminology of Advaita Vēdānta) living in a divine abode such as Vaikuṇṭha, Gōlōka or Kailāsa. There has been no scholarly refutation of Parādvaita from the point of view of Kēvalādvaita as far as I know, and some shoddy attempts at that use the above mistaken notion which Abhinavagupta vehemently refutes – that the Paramaśiva of Kashmir Shaivism is a form of Saguṇa Brahman. In fact, śrīvidyā school vehemently upholds this idea (as applied to Lalitāmbikā in this case) and reflected in the writings of Puṇyānandanātha, Bhāskararāya, and others.
Technicalities aside, authors like Mahāmahōpādhyāya Gopinath Kaviraj and M M Pundit point out that the difference between Advaita Vēdānta of Shankara and Parādvaita of Abhinavagupta and other Shaivite monists is more a difference of logic than of faith. Most Kashmir Shaivite authors point out that the belief and practice of Shankara and Gauḍapāda were those of theists and involving practices which can be classified as śāktōpāya. The shortcomings of Advaita Vēdānta are explained as derived from Buddhism, who the Advaitins were trying to refute and in the process, were greatly influenced by. Many modern Kashmir Shaivite authors point out the difference between the attitudes of classical Advaitins such as Gauḍapāda and Shankara (who were practice-oriented or adopting śāktōpāya skillfully) and the later ones like śrīharṣa and others who declared definite preference for logical debate over the spiritual practice. Similar to the case of post-Shankara Advaitins, Abhinavagupta also blames the later Buddhist logicians and the confusion caused by them rather than the original teachings of Lord Buddha.
As Gopinath Kaviraj states in one of his papers, a lot of misunderstanding regarding Trika Siddhānta arises from the fact that many of its modern-day writers and commentators are from traditional Advaita Vēdānta background. And their interpretation of Trika Siddhānta is heavily colored. So, yes, there are similarities, and there are some very obvious differences.