Guru: Literally means one who sheds light in the darkness. Guru is the teacher who sheds light of knowledge on the darkness of ignorance of a discipline.
Guru is one who performs the Deeksha to the worldy mundane human (bhavi), who receives another or second birth as it were, the spiritual birth, in contradiction to the physical birth in the world. And changed from ordinary human (bhavi) to worshipper (bhakta).
Guru who gives the devotee real insight into the principles of the religion explains the inner meaning of the practices and guides on the path of the final beatitude or final liberation or emancipation (Mukti). He provides the necessary instruction and training to enable bhavi to get through the arduous courses of spiritual discipline.
The Guru is the spiritual mother by his gracious look of the eyes, expressive of tenderness, just like the kind gracious look of the mother which calms down in distress and comforts her baby.
Guru: A Guru is one who is regarded as having great knowledge, wisdom, and authority in a certain area, and who uses it to guide others (teacher). Other forms of manifestation of this principle can include parents, school teachers, non-human objects (books) and even one's own intellectual discipline, if the aforementioned are in a guidance role.
Deeksha Guru: One who provides the Deeksha and provides blessings (Ashirwaad) with the Mantra chanting (Mantra Gopya). Deeksha Guru explains steps of Ishtalinga Puja. Deeksha Guru will be one for each Lingayat individual for getting the Deeksha for life time, in some other circumstances there may multiple Deeksha Gurus.
Siksha Guru: One who provides the literary knowledge (teacher), a person can learn from many teachers, all are treated as Siksha Guru .
Prabhudeva Praised his Guru Basavanna in the following Vachanas
He showed the entirety in Linga ,
he showed the light of that Linga in entirety,
By revealing the extraordinaire to the mind he saved me.
He disclosed himself within me, and he disclosed me within him.
Again, unifying the two, he became Guheshwara within me and prevailed as Mahalinga outside,
Him, that Sri Guru Linga Basavanna.
He exclaims that Basavanna is indeed the Guru of all, including GOD!
One who is aware that Shiva is Guru is indeed the Guru ;
One who is aware that Shiva is Linga is indeed the Guru ;
One who is aware that Shiva is Jangama is indeed the Guru ;
One who is aware that Shiva is Prasada is indeed the Guru ;
One who is aware that Shiva is Achara is indeed the Guru ;
Thus, being aware of the five-fold to be the five-bramhas,
Supreme soul Sangana Basavanna is the Guru to me,
Guru to you, and the Guru to the entire world, O Lord Guheshwara.
Shivayogi ಸಿದ್ಧರಾಮ ಶಿವಯೋಗಿ declares that he was a bewildered soul roaming from birth-to-birth subjected to the fruits of actions. It was Basavanna, as the Guru , who hoisted him from these endless cycles of births resulting from the laws of Karma. Having rescued him, Guru Basavanna further illumined the effective way of worshipping the Lord. Finally, when the self was dissolved in the blessed offering, Basavanna made him divine.
Basavanna is the Guru who hoisted the bewildered one,
subjected to the fruits of action.
Basavanna is the Guru who pointed at the right practice of revering the Almighty.
When everything was offered, he rendered me to be like him,
O Lord Kapilasiddhamallikarjuna.
 From the book "Vachana", pub: Basava Samiti Bangalore 2012.