Symbolism, Imagery, Allegory
Alan Paton uses these different Zulu forms of address for a couple of reasons: first, he wants to remind us that Kumalo and the other black characters in Cry, the Beloved Country come from a very specific cultural context, which is different from the background of white South African characters such as James Jarvis.
However, these names also symbolize different social relations based on respect in the novel. So, religiously minded people in Johannesburg like Mrs. Lithebe and everyone in Ndotsheni all call Kumalo umfundisi, which means reverend. When James Jarvis first addresses Kumalo as umfundisi (2.25.4), he proves (a) that he knows Zulu very well, a language that he probably would have learned on his farm; and (b) that he respects Kumalo's social position on his own terms, as a Zulu Christian priest. Jarvis's appreciation for Kumalo's status as an umfundisi indicates that Jarvis is open to trusting Kumalo, which is an important basis for their later working relationship.
When Kumalo calls Jarvis umnumzana—which means sir—in return, Kumalo shows Jarvis that he respects Jarvis's authority even in his own Zulu cultural context. None of the other white characters in the novel are called umnumzana; Kumalo saves this title for Jarvis because Jarvis is so generous to Kumalo even though Kumalo's son shot Jarvis's son. Similarly, when Jarvis's wife dies, Kumalo and the people of Ndotsheni call her inkosikazi, mistress, which shows their respect and humility to the entire Jarvis family. These honorable titles indicate that Kumalo is happy to show his gratitude towards this family that has helped his people so much.
Last but not least, there is the term inkosana. Kumalo uses this word to talk to James Jarvis's grandson; it means little master. Specifically, it's supposed to be for the son of a well-respected chief or gentleman. By calling the youngest Jarvis inkosana, Kumalo continues to show his respect towards the whole Jarvis family. But he also subtly encourages the boy to continue studying and learning Zulu. The youngest Jarvis's interest in the cultures of the black people who live around him makes a good sign that he will be a generous and liberal social reformer like his father and grandfather before him.
By the way, Tixo is a Xosa word that means "God." For more on Paton's use of this term, check out our "Quotes" section on "Power."