Molecular clouds are supported by turbulence and magnetic fields, but quantifying their influence on cloud lifecycle and star formation efficiency (SFE) remains an open question. We perform radiation MHD simulations of star-forming giant molecular clouds (GMCs) with UV radiation feedback, in which the propagation of UV radiation via ray-tracing is coupled to hydrogen photochemistry. We consider 10 GMC models that vary in either initial virial parameter ($1lealpha_{v,0}le 5$) or dimensionless mass-to-magnetic flux ratio (0.5-8 and $infty$); the initial mass $10^5M_{odot}$ and radius 20pc are fixed. Each model is run with five different initial turbulence realizations. In most models, the duration of star formation and the timescale for molecular gas removal (primarily by photoevaporation) are 4-8Myr. Both the final SFE ($epsilon_*$) and time-averaged SFE per freefall time ($epsilon_{ff}$) are reduced by strong turbulence and magnetic fields. The median $epsilon_*$ ranges between 2.1% and 9.5%. The median $epsilon_{ff}$ ranges between 1.0% and 8.0% and anticorrelates with $alpha_{v,0}$, in qualitative agreement with previous analytic theory and simulations. However, the time-dependent $alpha_{v}(t)$ and $epsilon_{ff,obs}(t)$ based on instantaneous gas properties and cluster luminosity are positively correlated due to rapid evolution, making observational validation of star formation theory difficult. Our median $epsilon_{ff,obs}(t)approx$ 2% is similar to observed values. We show that the traditional virial parameter estimates the true gravitational boundedness within a factor of 2 on average, but neglect of magnetic support and velocity anisotropy can sometimes produce large departures. Magnetically subcritical GMCs are unlikely to represent sites of massive star formation given their unrealistic columnar outflows, prolonged lifetime, and low escape fraction of radiation.
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