Excerpt taken from Station of Fear by Imaam Ibn al-Qayyim al-Jawziyyah:
The word khashya is more specific than khawf, for it is specific to the true knowers of Allah – as Allah, subhanahu wa ta’ala, said, “Truly, those who fear Allah from among His servants are the knowers.” [35:28] Hence, khashya. is fear associated with the intimate knowledge (ma’rifa) of Allah-as the Prophet, sallallahu alayhe wa sallam, said, “I am most mindful of Allah among you, and most intense in fearing Him.” (Bukhari, Muslim) Khawf is movement in its essence, while khashya is concentration, stillness, and holding of breath. For example, someone who sees a fierce enemy or a flood or something like that has two states: first, movement in order to flee from it, and this is the state of khawf. Second, his stillness in a place safe from the danger-and this is khashya.
Ar-Rahba means the urge to run away from the danger-which is the opposite of ar-Raghba, which means the urge of the heart to journey towards that which it likes.
Al-Wajal is the trembling of the heart upon the cognition or remembrance of someone whose power or punishment one fears.
Al-Haybah is fear associated with awe and glorification, and its greatest form is that which occurs in association with love and intimate knowledge (ma’rifa).
Al-Khawf, then, is for the common believers, while al-khashya is for the scholars with profound knowledge, while al-haybah is for those nearest to Allah. The extent of one’s fear for Allah is proportional to one’s knowledge, both formal and experiential, of Allah. As the Prophet, sallallahu alayhe wa sallam, said, “I am the most knowledgeable of Allah among you, and most intense in His khashya.” In another narration of the same hadeeth, the word used is khawf instead of khashya. The Prophet, sallallahu alayhe wa sallam, also said, “if you knew what I know, you would laugh little and weep much, and would not enjoy intimacy with women, and would go out wandering in the wildernesses and praying to Allah.” (Ahmad)
When faced with his object of fear, a man with khawf turns to fleeing and grabbing, while a man with khashya seeks the support of knowledge. For example, when a lay person is faced with an illness, he seeks to protect himself (and seeks someone who could help) while a skilled physician turns to investigating the illness and the cure.
Abu Hafs says, “Al-khawf is Allah’s lash with which He straightens up those fleeing from His door.” He also said, “Al-khawf is a lamp in the heart, with which the good and the evil inside of the heart can be seen-and everyone you fear from, you run away from him, except Allah-when you fear Him, you run towards Him for refuge.”
Hence, the one who fears Allah is a refugee towards His Lord’s [mercy] from his Lord’s [displeasure].
Abu Suleiman said, “Whenever fear (of Allah) departs a heart, it is ruined.” Ibraheem ibn Sufyan said, “When fear of Allah resides in hearts, it burns away the sources of lust and eradicates worldly attachments.” Thun-Noon said, “People will stay on the path so long as they have (Allah’s) fear when this fear leaves them, they will go astray.”
Fear, however, is not the end in itself, but a means towards an end. When that end, Allah’s ultimate pleasure, is attained, there is no need for fear. As Allah says to the people of Paradise: “there is no fear upon them, nor do they grieve.”
Read the rest here.