In this paper, we address how to provide absolute differentiated services for optical burst switching (OBS) networks based on dynamic wavelength assignment. Unlike existing Quality of Service (QoS), such as buffer-based and offset-time based schemes, our proposed Dynamic Virtual Lambda Partitioning (DVLP) scheme does not mandate any buffer or extra offset time, but can achieve absolute service differentiation between classes. This new DVLP scheme shares wavelength resources based on several different class priorities in an efficient and QoS guaranteed manner. Furthermore, DVLP is placed on robustness: this means that each traffic class with loss probability conforming to the target value continue to receive the required QoS, despite the presence of misbehaving packets such as bursty arrival traffics. We also state the conditions in which the DVLP is feasible. The feasible condition can be determined from the target loss probabilities that result with minimum loss probabilities based on the number of available wavelengths in each class. We then show the performance results of proposed scheme which effectively guarantees the target loss probability of each traffic class, both in long and short term periods.