The difference between a slave narrative and a neo-slave narrative could be compared to the difference between primary and secondary sources.
Based on personal experience, a slave narrative is a first hand testimony of slavery. As a primary source, slave narratives are not accounts written after the fact with the benefit of hindsight. A well-known example of a slave narrative. Narrative of the Life of Frederick Douglass is a slave narrative, because it was written during his lifetime, from Douglass' own personal experience.
Neo-slave narratives are fictional. Instead of being written from personal experience, these stories are created by contemporary authors who use historical information, research, and a healthy dose of imagination. Shirley Anne Williams' novel Dessa Roseor Middle Passageby Charles Johnson are both great examples of a neo-slave narrative.
Slave narratives, first written in the 18th century, detailed the accounts of actual slaves held captive in British colonies or in the United States. Slave narratives were often autobiographical, and many followed a traditional narrative of religious redemption; an example is The Interesting Narrative and the Life of Olaudah Equiano or Gustavus Vassa, the African, by Olaudah Equiano, published in London in 1789. The aim of slave narratives was to expose the horrors of slavery and argue for abolitionism, often using the argument that slavery was unchristian. An example is A Narrative of the Life of Frederick Douglass, an American Slave by Frederick Douglass, published in 1845.
Neo-slave narratives, on the other hand, were written starting in the 1960s and 1970s and include books such as Madison Smartt Bell's All Souls' Rising (1995), an account of the Haitian Revolution, Toni Morrison's Beloved (1987), and William Styron's Confessions of Nat Turner (1967). These narratives, unlike the earlier narratives, are novels rather than autobiographies, and their aim is not to argue for abolitionism but to provide novelistic insight and depth to the experience of slavery in the Americas. These novelistic accounts, written after the end of slavery in the Americas, are an attempt to grapple with the psychological realities and historical effects of the institution of slavery.