In data center servers, power management (PM) exploiting Dynamic Voltage and Frequency Scaling (DVFS) for processors can play a crucial role to improve energy efficiency. However, we observe that current PM policies (i.e., governors) not only considerably increase tail response time (i.e., violate a given Service Level Objective (SLO)) but also hurt energy efficiency. Tackling limitations of current PM governors, we propose NMAP, Network packet processing Mode-Aware Power management. NMAP improves energy efficiency while satisfying given SLOs, considering packet processing status on a core for PM by monitoring transitions between network packet processing modes - interrupt and polling. Tracking the transitions, NMAP detects moments that a core cannot process packets fast enough and forces the core to immediately raise the voltage and frequency (V/F) state. As a result, NMAP can provide not only low response time but also low energy consumption. Our experiment shows that NMAP improves tail response time by up to 4.1 compared with the ondemand governor, reducing energy by up to 44.6 percent compared with the performance governor.