HBV (Hepatitis B Virus) is primarily transmitted from infected blood exposure as in a blood transfusion, needle sharing of IV drugs, or mother to fetus during delivery.
Once the HBV is in the blood stream, it begins to replicate in the hepatocyte (liver cell). This replication interferes with liver function leading into acute hepatitis. Replication during this stage occurs when the virus enters the hepatocyte through endocytosis.
The virus's DNA is then transferred to the host's nucleus by way of the host's special protein called chaperones. Once inside the host's nucleus, the virus DNA is made into a full double stranded DNA through a critical enzyme called Reverse Transcriptase. Once it is a double stranded DNA, this allows the virus DNA to serve as a template to make RNA. One of the RNA's (mRNA) is transported back into the cytoplasm of the cell. Through this process and the enzyme Reverse Transcriptase the virus (virion life cycle continues).
Virion life cycles are more difficult than perhaps a parasites life cycle due to the mutations viruses can have.