  Blazars can be divided into two subtypes, flat spectrum radio quasars (FSRQs)
and BL Lac objects, which have been distinguished phenomenologically by the
strength of their optical emission lines, while their physical nature and
relationship are still not fully understood. In this paper, we focus on the
differences in their variability. We characterize the blazar variability using
the Ornstein-Uhlenbeck (OU) process, and investigate the features that are
discriminative for the two subtypes. We used optical photometric and
polarimetric data obtained with the 1.5-m Kanata telescope for 2008-2014. We
found that four features, namely the variation amplitude, characteristic
timescale, and non-stationarity of the variability obtained from the light
curves and the median of the degree of polarization (PD), are essential for
distinguishing between FSRQs and BL Lac objects. FSRQs are characterized by
rare and large flares, while the variability of BL Lac objects can be
reproduced with a stationary OU process with relatively small amplitudes. The
characteristics of the variability are governed not by the differences in the
jet structure between the subtypes, but by the peak frequency of the
synchrotron emission. This implies that the nature of the variation in the jets
is common in FSRQs and BL Lac objects. We found that BL Lac objects tend to
have high PD medians, which suggests that they have a stable polarization
component. FSRQs have no such component, possibly because of a strong Compton
cooling effect in sub-pc scale jets.
