1. Kabul--Kabul is the most significant of the three cities you mention in regards to the novel. In the first section of the novel set in 1970s Kabul, Hosseini describes a Kabul that is unrecognizable today. Business is booming in his original Kabul, and the West's influence on society and business is evident. Baba is able to get an abundance of food for celebrations or for day-to-day living, drive a new American car through the streets of the city, and build an orphanage. When Amir returns to Kabul, he feels certain that Farid has driven him to the wrong city. Hosseini writes through the eyes of Amir,
"Rubble and beggars. Everywhere I looked that was what I saw" (Hosseini 245).
In other words, "modern" Kabul is a foreign place to Amir. The only thing that remains unchanged is the smell of kabobs. The Soviets and then the Taliban have destroyed much of the city's infrastructure, and beggars are willing to sell prosthetic limbs so that they can eat. Baba's house is not in horrible condition, but it has been confiscated by Taliban-approved residents. In both his descriptions, Hosseini sticks close to a factual account of the change that occurred in Kabul. While at the writing of the novel, he had not had an opportunity to return to his home city, he interviewed Afghans who had lived in Taliban-controlled Kabul.
2. Peshawar--This Pakistani city appears twice in the novel. Amir and Baba stay in Peshawar after their escape from Afghanistan while they await their move to America. Amir returns years later to Peshawar to talk to Rahim Khan. As Amir rides in a taxi on his way to Rahim Khan's, he thinks that
"the bustle of [Peshawar] blurring past [him] reminded [him] of a busier, more crowded version of the Kabul [he] knew" (Hosseini 195).
While Afghanistan has regressed in time during Amir's life in America, Pakistan has progressed and offers Amir the smells and atmosphere of the city Baba loved so much--Kabul.
3. Islamabad--Near the novel's end, Amir must travel farther away from the Afghan border than Peshawar is to recover from his injuries. He goes to Islamabad, the capital of Pakistan. At the beginning of Chapter 24, Amir thinks,
"If Peshawar was the city that reminded me of what Kabul used to be, then Islamabad was the city Kabul could have become someday" (Hosseini 311).
Islamabad is bustling with civilization, progress, and culture. The mosques are in tact, and Amir and Sohrab stay in a decent hotel (especially compared to Amir's hotel in Kabul). Islamabad symbolizes what Baba, General Taheri, and Hassan dreamt Kabul could be.