The purpose of this discussion is to talk about how the performance has improved, not to raise issues with poor performance in the past.
Describe the achievement in detail – avoid vague comments.
“You’ve increased the client satisfaction rate to 93%” will motivate the employee much more than simply saying “I’m really pleased with how you’re going”.
With this approach you not only convey that you feel the improvement is important, you also convey you value the employee’s efforts which in turn provides encouragement for the employee to continue to improve performance.