Lightly grease the slow cooker with cooking spray.
Place the chicken breasts in the slow cooker.
Season the chicken with Cajun seasoning, smoked paprika, garlic powder, onion powder, salt, and black pepper.
Add the minced garlic, chicken broth, and diced tomatoes.
Cover and cook on LOW for 5 to 6 hours or HIGH for 2½ to 3 hours.
Remove the chicken and shred it using two forks.
Stir the softened cream cheese, heavy cream, and Parmesan cheese into the slow cooker until smooth.
Return the shredded chicken to the sauce and stir well.
Meanwhile, cook the penne pasta according to the package directions until al dente. Drain well.
Fold the cooked pasta into the slow cooker until evenly coated with the creamy Cajun sauce.
Cover and cook for another 10 minutes on LOW.
Garnish with chopped parsley before serving.