Heat oil in a saucepan over medium heat. Add curry paste and chicken and sauté for a minute. Pour in coconut milk, then add sugar, fish sauce and kaffir lime leaves and bring to a boil. Reduce heat to a simmer and cook for 15 to 20 minutes, until sauce starts to coat the chicken. Add beans and half the basil and cook, stirring occasionally, for 2 to 3 minutes, until the beans are tender and the chicken is cooked through. Cover and keep warm on low heat.