This systematic literature review examines security challenges and penetration testing mitigations within the Internet of Vehicles (IoV), integrated with 5G technology. While IoV improves traffic control, road safety and sustainable transportation, it incurs serious cybersecurity vulnerabilities. Cyber threats include Denial of Service (DoS), Man in the Middle (MitM), and eavesdropping are very common in this paradigm. They can eventually compromise the integrity, confidentiality, and availability of vehicular data, leading to traffic accidents and data leakage. According to Kitchenham guidelines, this review follows an extensive search about peer-reviewed articles in the past 5 years. This synthesizes the findings to reveal critical vulnerabilities in the IoV systems, more particularly those it exacerbates by 5G technology, and underscores the importance of penetration testing. It is an important way of identifying and fixing the system vulnerabilities through a simulated cyberattack within a controlled environment before such vulnerabilities may be used by malevolent actors. Effective strategies are also reviewed, such as compliance testing, machine learning for anomaly detection, and advanced model-checking tools such as Scyther and Tamarin. The review also explores current penetration testing practices updated to address dynamic cyber threats. Case studies, such as security and privacy issues of 5G-enabled autonomous platoons, demonstrate practical applications. Suggestions for future research include enhancing threat detection and prediction techniques, developing security frameworks tailored to IoV environments, and fostering cooperation between academia and industry. This SLR is a valuable resource for researchers and practitioners interested in securing IoV systems against evolving cyber threats, particularly as these systems advance with 5G technology.